Apparently this could be caused by some application locking the clipboard. If you are running some kind of remote desktop connection when the problem occurs, try closing it, since they seem to be the primary suspects for this behavior.
Someone in <a href=”http://www.howtofixcomputers.com/bb/ftopic140408.html”>this thread</a> suggested to download and run an application called <a href=”http://windowsxp.mvps.org/temp/GetOpenClipboardWindow.zip”>GetOpenClipboardWindow</a> whose objective is try to open and close the clipboard and report any problems it may encounter.
I have not tested it. Be careful if you decide to use it.