How to screenshot part of the screen under windows?

If they are on Vista or Win7, there is a new Snipping Tool that does exactly what you describe included. Just type "Snipping Tool" in the start menu search bar and it will open up.

Available in:
Snipping Tool is available only in the Home Premium, Professional, Ultimate, and Enterprise editions of Windows Vista/7.

** I just ran across this little gem which adds a snipping tool for XP users. http://xpsnipping.codeplex.com/