If you copy a few of those files to another pc (and then to that pc's
harddrive), can that pc open your files?

I'd try to determine if it was my copy of excel on my pc or something wrong with
the workbook itself.

If it turns out that it's your pc/excel, then depending on your version of
excel/windows, maybe you could try to completely remove excel, then reinstall.

OFF2000: Utility to Completely Remove Remaining Office CD1 Files and
        Registry Entries
http://support.microsoft.com/?kbid=239938

(and there's one for xl97, too)
OFF97: How to Completely Remove Microsoft Office 97
http://support.microsoft.com/default.aspx?scid=kb;EN-US;158658

(Some versions of windows won't allow this and I don't recall seeing a KB
article for xl2002+.)

If the workbooks themselves can't be opened on any pc, then...

Sometimes opening and saving in a different (newer?) version may help.  
Sometimes not.

If that didn't help (or if it's just one workbook), maybe opening the workbook
in OpenOffice and saving there would clean it up enough for excel to accept it.

http://www.openoffice.org, a 60-65 meg download or a CD

If it's just one file and the file is really important, there are commercial
recovery services.  I've never used it, but you might want to check into:
http://www.officerecovery.com

If you are using XL2000 or XL97 then it may be your toolbar customisation
file is getting close to blowing up on you. If so then go look for a file
called *.xlb and either rename it to *.old or just
delete it. NOTE:- This is your toolbar customisation file and you will lose
any customisations as a result of doing this. Excel will simply create
another one for you. If you have just renamed it and this was not the
problem then you can if you wish just go find the new *.xlb and delete it
and then rename the *.old back to *.xlb

Regards
              Ken............................