Keyboard shortcuts are usually entered as a combination of 2 or more keys that, when pressed simultaneously, perform some action. 

Windows Key shortcuts press and hold the key with the Windows logo on it and then press the following key:
F : Opens the search dialog
R : Opens the Run dialog
U : Opens Utility Manager (screen reader etc) dialog
E : Opens the My Computer Window
D : Displays the Desktop (minimises all open windows)
L : Switches user to the logon screen – Useful if you use a password and want to quickly lock your machine.
B : Displays the start menu bar
M : Minimises all windows to display the desktop

Windows system shortcuts.  Most Windows-based applications use these.
F1 : Help
CTRL + ESC : Open Start menu
ALT + TAB : Switch between open programs
ALT + F4 : Quit program
SHIFT + DELETE : Delete item permanently
CTRL + C : Copy
CTRL + X : Cut
CTRL + V : Paste
CTRL + Z : Undo
CTRL + B : Bold
CTRL + U : Underline
CTRL + I : Italic

Mouse click/keyboard modifier combinations for shell objects
SHIFT + right click : Displays a shortcut menu containing alternative commands
SHIFT + double click : Runs the alternate default command (the second item on the menu)
ALT + double click : Displays properties
SHIFT + DELETE : Deletes an item immediately without placing it in the Recycle Bin

General keyboard-only commands
F1 : Starts Windows Help
F10 : Activates menu bar options
SHIFT + F10 : Opens a shortcut menu for the selected item (this is the same as right-clicking an object
CTRL + ESC : Opens the Start menu (use the ARROW keys to select an item)
CTRL + ESC or ESC: Selects the Start button (press TAB to select the taskbar, or press SHIFT+F10 for a context menu)
CTRL + SHIFT + ESC : Opens Windows Task Manager
ALT + DOWN ARROW : Opens a drop-down list box
ALT + TAB : Switch to another running program (hold down the ALT key and then press the TAB key to view the task-switching window)
SHIFT : Press and hold down the SHIFT key while you insert a CD-ROM to bypass the automatic-run feature
ALT + SPACE : Displays the main window’s System menu (from the System menu, you can restore, move, resize, minimize, maximize, or close the window)
ALT + – (ALT+hyphen) : Displays the Multiple Document Interface (MDI) child window’s System menu (from the MDI child window’s System menu, you can restore, move, resize, minimize, maximize, or close the child window)
CTRL + TAB : Switch to the next child window of a Multiple Document Interface (MDI) program
ALT + underlined letter in menu: Opens the menu
ALT + F4 : Closes the current window
CTRL + F4 : Closes the current Multiple Document Interface (MDI) window
ALT + F6 : Switch between multiple windows in the same program (for example, when the Notepad Find dialog box is displayed, ALT + F6 switches between the Find dialog box and the main Notepad window)

General folder/Windows Explorer shortcuts
F2 : Rename object
F3 : Find all files
F4 : Selects the Go To A Different Folder box and moves down the entries in the box (if the toolbar is active in Windows Explorer)
F5 : Refreshes the current window.
F6 : Moves among panes in Windows Explorer
CTRL + X : Cut
CTRL + C : Copy
CTRL + V : Paste
CTRL + G : Opens the Go To Folder tool (in Windows 95 Windows Explorer only)
CTRL + Z : Undo the last command
CTRL + A : Select all the items in the current window
SHIFT + DELETE : Delete selection immediately, without moving the item to the Recycle Bin
ALT + ENTER : Open the properties for the selected object

Windows Explorer tree control
Numeric Keypad + * : Expands everything under the current selection
Numeric Keypad + + : Expands the current selection
Numeric Keypad + – : Collapses the current selection.
RIGHT ARROW : Expands the current selection if it is not expanded, otherwise goes to the first child
LEFT ARROW : Collapses the current selection if it is expanded, otherwise goes to the parent

New Windows 7 Keyboard shortcuts
Win + Space : operates as a keyboard shortcut for Aero Peek which hides all application windows so that you can see the desktop
Win + Up and Win + Down : are new shortcuts for Maximize and Restore/Minimize.
Win + Shift + Up : vertically maximises the current window
Win + Left and Win + Right : snap the current window to the left or right half of the current display; successive keypresses will move the window to other monitors in a multi-monitor configuration.
Win + Left and Win + Right : move the current window to the left or right display.
Win + + and Win + – (minus sign) : zoom the desktop in and out.
Win + Home : operates as a keyboard shortcut for Aero Shake.
Win + P : shows an “external display options” selector that gives the user the choice of showing the desktop on only the computer’s screen, only the external display, on both at the same time (mirroring), or on both displays with independent desktops (extending).

Windows 7 Taskbar
Shift + Click, or Middle click : starts a new instance of the application, regardless of whether it’s already running.
Ctrl + Shift + Click : starts a new instance with Administrator privileges; by default, a User Account Control prompt will be displayed.
Shift + Right-click : shows the classic Window menu (Restore / Minimize / Move / etc); right-clicking on the application’s thumbnail image will also show this menu. If the icon being clicked on is a grouped icon, the classic menu with Restore All / Minimize All / Close All menu is shown.
Ctrl + Click on a grouped icon : cycles between the windows (or tabs) in the group.

Related posts:

1. How to place shortcuts on your Quick Launch Toolbar
2. How to force Windows Explorer to open up and display in a pre-selected folder