|Three more things you could also try:|
Start Excel in Safe Mode,
This starts Excel as if it were the first time after Setup.
Type in: C:\Program Files\Microsoft Office\Office\Excel.exe /s
This will bypass:
all files in any Excel startup folder,
all Add-ins ( the addins will display but are not loaded.)
Only the original installation default toolbar setting is used and any Excel specific registry information, with only the original default registry settings are used.
That may help you track down the culprit.
You could also try:
First run the /unregserver to have Excel unregister itself.
Then run /regserver to have it rewrite all of its registry keys and re-associate itself with all of the Excel files.
Just as an afterthought, I would imagine that you have already checked the Shortcut properties to make sure Excel is being started correctly, not with some odd combination of options.