Written by Neal Axton:
There’s a screen capture utility in Windows Vista and Windows 7 under START -> PROGRAMS -> ACCESSORIES -> SNIPPING TOOL.
You can also use the PRINT SCREEN button on your keyboard and then the free paint program on most PCs.
Here are directions on screen grabs with Mac OS X.
A free utility I’ve used before is MWsnap.
I own Snag-It and use it extensively, it has a free 30 day trial.
There are also a lot of free utilities that download into browsers to do screengrabs. Here’s a list of 6 for Firefox (since I know you have to use it for Zotero anyway).