First are you running SP2 for Office 2007 - choose Office Button, Excel Options, Resources and see if there is a reference to SP2 on the last line.
Also here are some of the suggestions from other threads:
Search for excel.xlb, *.qat and *.pip. Try searching and renaming this files. Check if excel works.
Then check these:
http://support.microsoft.com/kb/938538
http://support.microsoft.com/kb/924611
Try booting Excel in safe mode and see if that helps: http://support.microsoft.com/kb/929135
If still it doesn't work than, Click on Start > Contorl panel> programs > programs & features> Select office 2007> Change> click on drop down of excel and make it NOT AVAILABLE.
Click on Start > Run > regedit > hit Enter
Take back and then delete the EXCEL FOLDERS under following registry keys:
Browse to HKEY_CURRENT_USER\Software\Microsoft\Office
Browse to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office
Browse to HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Office
Start> control panel> programs> programs & features> highlight office 2007> change> click on drop down of excel and make it AVAILABLE.
Warning - modifying the registry can cause serious problems if not done correctly - so you should make a backup first.
If this answer solves your problem, please check, Mark as Answered. If this answer helps, please click the Vote as Helpful button. Cheers Shane Devenshire