Someone said :
If you have Windows 7 you will have a program called "snip" this lets you capture your screen and save it to file.
Shift+PrtSc works too, but it copies your screen to your clip-board. You then have to paste this to somewhere. So you can, for example, open MS Word and then paste your screen in to a Word doc.
I tried snip and I did not like it.
When it stips working, for no reason, after a number of snips, it stops the normal copy/paste from working too! so I end up having to reboot the machine.
It may have been my machine but I never used it again...
I prefer to print screen (you can use Alt + Print Screen to print only the active window) and paste in "Paint", then edit and copy what I need.
if you just "paste" the result into a document or email, the resulting document size may be too big due to the Bitmap format likely to be used to store the image...
So try various "paste as" in MS Word for instance, to find a paste format that respects your image but does not make it too big.
You have more "paste as" options when you paste from Paint than directly from a screen grab.