I had the same problem. I followed the advice below and it was fixed:
(adapted from http://www.officearticles.com/excel/troubleshooting_microsoft_excel.htm)
What we're doing is removing Excel from the Windows registry. You don't need to enter values in the registry. When you launch Excel again, it will re-register itself, so you can expect it to take a little longer to launch than usual.
The re-registry process may reset some user options to factory values, so you may have to adjust Excel?s look and feel to your liking, using the normal Excel controls, after this is done.
So, let's go...
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Left-click Start, then Run, and type ?regedit? (without the quotes) and hit Enter. Browse, just like you would in Windows Explorer or My Computer, to the following folder:
My Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\"X.X"\Excel
"X.X" will be as follows, depending on your version of Word:
? Excel 2000 is 9.0
? Excel 2002/XP is 10.0
? Excel 2003 is 11.0
? Excel 2007 is 12.0
Once you find it, right click the Excel folder, and choose Rename. Rename it to OldExcel. (This is a quickie method of backing up your registry.) Close the Registry Editor.
Launch Excel. The re-registry process will put up a couple of information screens and Excel will eventually start. This should take care of the problem.