...or anything else for that matter, on a PC.
Step one:hit "Ctrl"+"PrtSc"
That is it.
You will have a screen capture in your clipboard. If you want to edit it, paste it into your favorite editor, such as Paint.Net (an excellent free editor).
Print screen is a great way to get pics from sites that won't let you right click to save.
I use Irfanview as my image editor/viewer.
If you have Windows 7, use the 'Snipping Tool' in the Accessories. This allows you to select the exact part of the screen you want, and to save it as a jpeg file. There are similar free downloads for XP users.
Ctrl-Prtsc copies the whole screen and all open windows.
Alt-Prtsc copies just the active window.
The snipping tool in Windows 7 gives you much better choice though.