Having just recently converted to Windows Vista (Home Premium 64-bit; it came pre-installed on my new computer), I’m still trying to (stay positive and) learn about all of its new features (if you’re looking for a Vista h8 post, you’ve come to the wrong place).
One of the best tools (also available in Windows 7) that I’ve come across is its built-in Snipping Tool, which is essentially an evolution of the old-school PrtScn function. Unlike the latter which only captures your entire screen (thus the name), Snipping Tool offers four different “snip” types: free-form (allows you to capture any shape region), rectangular, window, and full-screen (the last 3 are self-explanatory, I hope 
).
To find/try this great tool, go to your Vista start menu search box and type “snipping tool.” Still using XP? No problem (def. can’t blame ya), freeware PrtScr (notice it ends in an “r” not “n”) offers the same functionality.
FYI Alt+PrintScn in XP will copy just the window to the clipboard. The more you know!
sorry, can’t resist, but Mac OS X has been able to do the free form capture (in addition to full screen) for at least the last two versions, but possibly more, i.e. 4+ years. Command+Shift+4 for free form, +3 for full screen. Creates a png file on the desktop.
thanks! this is very helpful.