Does it happen when you're pasting things into Photoshop, or every program?
There is a known bug with memory frangmentation and Photoshop, because Photoshop actually uses an independant clipboard to the Windows one, and simply copies what's on your Windows one across. Ocassionally this system fails, and you cannot paste screenshots.
If that is the case for you, you can find a fix here:
http://scrapstreet.blogspot.com/2008...toshop-cs.html