Error Message 'Cannot empty clipboard'

I have Office 2007 installed in my computer and just now when I tried to copy and paste within Excel, and also within Outlook and also from Outlook to Word, I got an error message stating that the Clipboard could not be emptied. When I clicked OK, then the message went away and I was able to paste, but the message appeared again, each time when I tried to copy. So, I just restarted the computer and then issue didnt occur anymore. Is there anyone who knows why this thing happens? Thanks.

The clipboard problem occur occasionally in Office 2003 as well, so the only way to solve this issue is to logoff and then logon back again.