Both Windows XP and MAC OS X have various options for screen capturing that are very useful.

Entire Desktop (including dual screens)

XP -  Press Print Screen to capture the entire desktop to your clipboard for you to paste into another program. Press Cltr + V to paste.

OS X - Press Command-Shift-3. The screen shot will be automatically saved as a PNG file on your desktop.

Press Command-Control-Shift-3. The screen shot will be placed on your clipboard for you to paste into another program.

Selected Window

XP - Hold Alt and press Print Screen to capture the selected window to your clipboard for you to paste into another program.  Press Cltr + V to paste.

OS X -  Press Command-Shift-4, then press the Spacebar. When you have the cursor over a window you want to capture, just click the mouse button and the screen shot will be saved as a PNG file on your desktop.

Add Control to the shortcut above to place the screen shot on the clipboard instead of saving it to the desktop.

 Select A Region Using Rectangular Selection

XP -  Click on the Snippy icon , then hold down Shift and click and drag around the area you want to capture.  Press Cltr + V to paste.
*must install Snippy program (see below)

OS X - Press Command-Shift-4 then click and drag to select the area you wish to capture. When you release the mouse button, the screen shot will be automatically saved as a PNG file on your desktop.

Add Control to the shortcut above to place the screen shot on the clipboard instead of saving it to the desktop.

Special OS Specific Capture Options

XP -  To select a region using freehand selection, click on the Snippy icon, then click and drag around the area you want to capture.  Press Cltr + V to paste.
*must install Snippy program (see below)

OS X -  You can also use the Grab utility, located in the Applications > Utilities folder.  Grab lets you include a cursor or a menu, save in another format, and do a timed screen capture.

Snippy Installation Instructions for XP

1. Download the “snippy.exe” file and bring it to your local computer (desktop or my documents works just fine). You can get it from: http://www.bhelpuri.net/Snippy/SnippyDownload.htm
2. There is no installation, you just double click on “snippy.exe” each time you want to run the program and the icon will appear in your taskbar