there are programs out there that will do it for you, but i do it the old fashioned way:
press and hold SHIFT, then press the Print Screen Key, (usually upper right hand corner of your keyboard)
open Windows Paint, (start, programs, accessories, Paint) i made a shortcut to paint on my start menu to save time.
once Paint is open, right click on the blank document and choose "Paste".
from there you can cut the area out you want to share using paint's little square edit thingy, cut and paste into a new document and save to your hard drive.
sounds complicated, but after you do it a few times, it's easy :-)