Not trying to contradict previous message at all. You might have other problems and a system restore might help. If however you find that after copying and pasting a lot the problem recurs then try the following.
It depends how much copying and pasting you are doing. When you copy anything and paste it. It goes to clipboard and stays there in your PC's memory. Eventually clipboard will be full and unable to hold more text/images. When that happens you can copy but u wont be able to paste. Now the other problem with extensive copy and paste is that it will slow down your PC as clipboard store information in your PC's memory. The best simple solution is to get a small program called Hot Copy Paste. Once clicked it will clear the clipboard and u should be fine. Hot Copy Paste can be downloaded here.