I have a similar problem but the unregister register didn't fix it. neither
did the Ignore other applications.
My problem may be related to a previous install and removal of Office 2007.
I had numerous problems and even program crashes with 2007 so finally gave up.
I removed Office 2007. I even followed the MS kb
http://support.microsoft.com/kb/928218 to edit the registry in an effort to
remove the offending 2007.
Did a complete fresh install of 2003 but dbl click of a file will start
Excell but not open the file. If I then select OPEN a file and navigate to
the file in question it will open and all appears OK.
Suggestions appreciated.