There are several ways in which a virus can infect your system.  Infected email attachments is the most common source of infection. These attachement may be an image, a document or just about any other type of file that can be attached to an email. Infections are also spread through internet file sharing and download sites.

Types of Threats

• Worm – a self-replicating program which uses the system to send copies itself to other systems with no user intervention
• Spyware – a program that becomes installed on a system which collects small bits of information about the system or user without their knowledge
• Adware – a program that displays or downloads advertisements to a PC.  The ads may appear as a popup, or may appear in a portion of the web browser
• Malware – a broad term for a wide range of malicious software whose intent is to disrupt your system, gather information about the system or gain access to system resources. Malware is not necessarily the virus but is often the method of transmittal
• Trojan Horse – a program that masquerades as a safe application yet contains malicious code that will cause an infection.
• Rogue Software – decieving or misleading programs that trick users into paying for an unnecessary or fake security software

Antivirus Software

Programs that are designed to detect and remove or neutralize viruses are referred to as Antivirus Software.  They work by periodically scanning your system for known viruses and also by monitoring all new files being added to your system.

How Can You Tell If Your PC Is Infected

The existence of one or more of the following may be an indication that your system is infected:

• Random pop-ups- Unknown files or icons appearing on your desktop or other places when you did not install them
• Files or icons disappearing from your system without you having uninstalled or moved them
• Unusual error messages
• Hardware devices (hard drives, CD Drives, etc) can no longer be accessed
• Applications crash unexpectedly, run slowly or don’t start up at all
• Your system becomes unstable and freezes or crashes frequently
• Your system will not boot up at all

Virus Prevention

Prevention is the best approach to maintaining a healthy system.  Installing a reputable Antivirus program is your first priority.  The program you install must update itself frequently with files that contain lists and characteristics of the most recent viruses that are circlating.  In addition to the Antivirus program, maintain healthy habits with regard to your online and email activities.- Never download attachments from email accounts that you do not know.

• Only download files from trusted sites
• ensure that you have a good firewall between your network and the internet
• update your PC with the latest updates as they are released

Now available for download, Microsoft Security Essentials (MSE) is a comprehensive Antivirus and Antispyware solution to keeping your PC safe from attacks.

This free application replaces Windows Defender and is a strong competitor for current antivirus applications. It has been in Beta release since June and has received mostly positive reviews.

• more information at Windows 7 News

If you are looking for a free solution to combat both Spyware and Viruses, consider this Microsoft offering. It is highly recommended.

/doc

- Updated Antivirus Program
- Updated Windows Defender
- Updated Common Sense

1.  Anti Virus Program
Links are provided on the main page to two great, free antivirus programs.  The main reasons I like these so much is that they don’t consume a lot of system resources and they are continuously updated.  It’s basically a set and forget situation for ether one.

A quality antivirus program makes sure that your PC can detect and remove *most* threats that come your way.

UPDATE:  Read about Microsoft Security Essentials here

2.  Windows Defender
Windows Defender protects your PC against spyware, malware and adware which can cause your PC to run erratically or even crash by detecting and removing known spyware from your PC.  It also features real-time protection which works while you are online.  Windows Defender updates itself very frequently through the Microsoft Update process.

Windows Defender is included with Vista and WIN7.  It is available for XP here http://www.microsoft.com/downloads/details.aspx?FamilyId=435BFCE7-DA2B-4A6A-AFA4-F7F14E605A0D&displaylang=en&mg_id=10134

UPDATE:  Read about Microsoft Security Essentials here

3.  Common Sense
You have to keep your eyes wide open to all possible sources of attacks on your PC.  Threats can come from almost anywhere, but are mostly concentrated around certain types of sites.  Sites to be cautious of are:

Online Gaming sites
File-Sharing sites
Social Networking sites
Adult content sites
etc.

Be careful when any window pops up asking you to click something. One of the most pervasive techniques that hackers use is to pop up a window that says that your PC has a virus and that you must “click here” to remove it.  And when you click it, you open youself up to whatever is lurking.  Sometimes, there will even be a button that looks like a common ‘close’ button, but in reality is another gotcha.  To be safe, use the keyboard shortcut Alt-F4 to close the window safely.

Sometimes, even popular programs contain an option to also install a companion or affiliate program that can slow down your PC by monitoring your browsing habits in order to suggest similar sites that might interest you.

• Here is a great article that discusses 6 essential tips to stay safe while online.

If you practice safe browsing habits and have good anti-virus/anti-spyware protection, the chances of your being attacked are greatly reduced.

/doc

Introduced in January of 2005,  Windows Defender is Microsoft’s free anti-spyware application.  It is included free with Windows Vista and Windows 7, and is available as a free download for Windows XP. 

It was announced in December of 2008 that Windows Defender is being discontinued and will be replaced by a new application known as Microsoft Security Essentials (MSSE).  MSSE is currently in Beta release and is expected to be released in November of 2009. 

More info on Windows Defender:
http://www.microsoft.com/windowsxp/using/security/expert/glenn_windefender.mspx

/doc

If your computer starts to behave strangely or displays any of the symptoms listed below, you may have spyware or other unwanted software installed on your computer.

I see pop-up advertisements all the time
Some unwanted software will bombard you with pop-up ads that aren’t related to a particular website you’re visiting. These ads are often for adult or other websites you may find objectionable. If you see pop-up ads as soon as you turn on your computer or when you’re not even browsing the web, you may have spyware or other unwanted software on your computer.

My settings have changed and I can’t change them back to the way they were
Some unwanted software has the ability to change your home page or search page settings. This means that the page that opens first when you start your Internet browser or the page that appears when you select "search" may be pages that you do not recognize. Even if you know how to adjust these settings, you may find that they revert back every time you restart your computer.

My web browser contains additional components that I don’t remember downloading
Spyware and other unwanted software can add additional toolbars to your web browser that you don’t want or need. Even if you know how to remove these toolbars, they may return each time you restart your computer.

My computer seems sluggish
Spyware and other unwanted software are not necessarily designed to be efficient. The resources these programs use to track your activities and deliver advertisements can slow down your computer and errors in the software can make your computer crash. If you notice a sudden increase in the number of times a certain program crashes, or if your computer is slower than normal at performing routine tasks, you may have spyware or other unwanted software on your machine.

 http://www.microsoft.com/windows/products/winfamily/defender/default.mspx

Spyware is often associated with software that displays advertisements (called adware) or software that tracks personal or sensitive information. Not all software that provides ads or tracks your online activities is bad. For example, you might sign up for a free music service, but you "pay" for the service by agreeing to receive targeted ads. If you understand the terms and agree to them, you may have decided that it is a fair tradeoff. You might also agree to let the company track your online activities to determine which ads to show you.

Other kinds of spyware make changes to your computer that can be annoying and can cause your computer to slow down or stop responding. These programs can change your web browser’s home page or search page, or add additional components to your browser you don’t need or want. These programs also make it difficult for you to change your settings back to the way you originally had them.

There are a number of ways spyware or other unwanted software can get on your computer. A common trick is to covertly install the software during the installation of other software you want such as a music or video file sharing program. The key in all cases is whether or not you (or someone who uses your computer) understand what the software will do and have agreed to install the software on your computer.

Whenever you install something on your computer, make sure you carefully read all disclosures, including the license agreement and privacy statement. Sometimes the inclusion of unwanted software in a given software installation is documented, but it might appear at the end of a license agreement or privacy statement.

http://www.microsoft.com/windows/products/winfamily/defender/default.mspx