Press the "C" key or go to Options > Capture/Screenshot. You can set the options, including the hotkey (CTRL+F11 by default) in the dialog that appears. Press the Start button and Irfanview will minimise. When you want to take the screenshot press the hotkey and the program will appear with your screenshot.

Dunno about Faststone. Do you mean the image viewer? I think they also make a specialist screen capture program. Check their Web site.

EDIT: Yes, they do have an image capture program. See http://www.faststone.org/FSCaptureDetail.htm

ANOTHER EDIT: Faststone Capture is shareware, but it seems from the image viewer program page that it also has screen capture abilities. See http://www.faststone.org/FSViewerDetail.htm

The Snipping Tool only appears in Vista (and probably also Windows 7).