Are you running Skype? This has been the best solution I have found to get rid of the "cannot empty the clipboard error" in Excel 2007 & 2010. Delete the Skype add-on in IE and/or Firefox and good-bye annoying error!
In reply to rjacobs7 post on February 28, 2011
Cannot clear clipboard error - Windows 7, Excel 2010 -
This error occurs nearly every time a drag and drop of cell contents is attempted. I've had this same error over the past 10 years on older computers and older versions of Windows and Office. It has now reoccurred with a new laptop running Windows 7 64 bit and Office 2010. The issue can be replicated only if a browser - IE or Firefox - is open at the same time that Excel is open. Having Word and/or Outlook open at the same time will not cause the problem to occur unless a browser is also open. This error is extremely irritating and no solutions from Microsoft or other posts on this issue resolve it.
I have a solution - at least for me! Delete the Skype add-in in IE and Firefox and the "cannot clear clipboard" error after a drag and drop goes away when IE and/or Firefox are running. Apparently some sort of memory-management issue with Skype, Office and the browsers.