Windows Clipboard Not Working...

My windows clipboard just randomly stops working. It was working fine until a few weeks ago, and now it stops working for hours on end. Just now, however, I'm starting to get error messages too, its the typical windows error popup with the red x stopsign thing that just says "cannot open clipboard." I'm running Windows XP Professional and I've never had this problem before, and until I got the error message I thought it was just my keyboard crapping out on me. Help please