Well, there are actually two ways of capturing windows or scrren areas: it all depends on the operating system you are using.

1. I shall start with Windows XP (Home or Professional): in order to capture your screen you need to press the 'PrtScn'(print screen) key on your keyboard. This will copy the entire display into the clipboard. Next, you can open an image editor such as MS Paint and perform a 'Paste' operation (you can use the 'Ctrl+V' key combination or go to the menu to 'Edit' -> 'Paste') in order to get the image into the the image editor. Now you can edit it and save it to a location of choice. 

If you wish to capture an active window (a program window), you can press 'Alt+PrtScn' and then paste the image into the image editor. The same process can be followed in order to capture your screen in Windows Vista Home Basic.


2. If you are using Windows Vista (other than Windows Vista Home Basic) or Windows 7 (seven), you can use the Snipping Tool. This can be found in Start -> All Programs -> Accessories.

The Snipping Tools is quite easy to use: once started, you can drag the mouse over your screen in order to create a rectangle to be captured (you can  capture the entire screen, a smaller area or active windows).The Snipping tool works the same both in Vista and in Win7. The screenshots are done in Windows 7, but there's no difference.

After selecting the screen area you wish to capture, you have some basic options for editing. Saving is quite a simple thing. Just go to File -> Save as, select the graphics format (PNG, JPG, GIF or MHT - single HTML file), enter a name for your capture and save it to a location of choice. Good luck capturing!