Has the cat jumped on your keyboard and flipped the screen upside down, or rotated it? Ctrl + Alt + [One of the Arrow Keys] will fix this.

Below is a list of some other helpful shortcuts. The → symbol stands for the right arrow key, ← for the left arrow key, etc.

New in Windows 7
Win+↑ Maximize the current window
Win+↓ If the current window is maximized, restore it; if the current window is restored, minimize it
Win+← Dock the current window to the left half of the screen
*If it is already docked left, it is moved to the right half of the screen
*If it is already docked right, it is restored to its original size
Win+→ Dock the current window to the right half of the screen
*If it is already docked right, it is moved to the left half of the screen
*If it is already docked left, it is restored to its original size
Win+Shift+← Move current window to the left monitor (with dual monitors)
Win+Shift+→ Move current window to the right monitor (with dual monitors)
Win+Home Minimize all but the current window
Win+Space Peek at the desktop
Win+[Plus sign] Zoom in
Win+[Minus sign] Zoom out
Win+P Open the projection menu (generally used for laptops connected to projectors)
Alt+P In Explorer, show/hide the preview pane

Taskbar Modifiers (New in Windows 7)
Shift+Click Open a new instance of the program
Ctrl+Click Cycle between windows in a group
Middle Click Open a new instance of the program
Ctrl+Shift+Click Open a new instance of the program as Administrator
Shift+Right-Click Show window menu

Managing Windows
Alt+F4 Close the active window
Alt+Tab Switch to previous active window
Alt+Esc Cycle through all open windows
Win+Tab Flip 3D
Ctrl+Win+Tab Persistent Flip 3D
Win+T Cycle through applications on taskbar (showing its live preview)
Win+M Minimize all open windows
Win+Shift+M Undo all window minimization
Win+D Toggle showing the desktop
Win+↑ Maximize the current window
Win+↓ If the current window is maximized, restore it; if the current window is restored, minimize it
Win+← Dock the current window to the left half of the screen
*If it is already docked left, it is moved to the right half of the screen
*If it is already docked right, it is restored to its original size
Win+→ Dock the current window to the right half of the screen
*If it is already docked right, it is moved to the left half of the screen
*If it is already docked left, it is restored to its original size
Win+Shift+← Move current window to the left monitor (with dual monitors)
Win+Shift+→ Move current window to the right monitor (with dual monitors)
Win+Home Minimize all but the current window
Win+Space Peek at the desktop
Win+[Plus sign] Zoom in
Win+[Minus sign] Zoom out

Starting Programs
Win+1 Open the first program on your Quick Launch bar
Win+2 Open the second program on your Quick Launch bar
Win+n Open the nth program on your Quick Launch bar
Win+U Open the ease of access center
Win+F Open the search window
Win+X Open the Mobility Center
Win+E Open Explorer
Win+R Open the Run window
Win+B Move focus to notification tray (the right-most portion of the taskbar)
Win+P Open the projection menu (generally used for laptops connected to projectors)
Win+Pause Open the System Properties portion from the Control Panel
Ctrl+Shift+Esc Open Windows Task Manager

Logging In And Out
While the below shortcuts seem unwieldy because of their length, they’re quite easy to remember once you try them out a few times.
Win, →, Enter Shutdown
Win, →, →, R Restart
Win, →, →, S Sleep
Win, →, →, W Switch Users
Win+L Locks computer

Viewing Folders With Explorer
Alt+← Go back
Alt+→ Go forward
Alt+↑ Go up a directory
Alt+D Move focus to address bar
Alt+D, Tab Move focus to search bar
Alt+Enter Open the Properties window of the current selection
Ctrl+Mousewheel Change the view type (extra large, small, list view, detail, etc.)
Alt+P Show/hide the preview pane