One of the things that excel does when it's closing is to update your customized
toolbar file. Maybe that file has gotten corrupted and is causing errors with
excel.
Use windows start button|Find|to search for *.xlb. (Name and location vary with
version of windows and excel.) (and you might have to search through hidden
folders for hidden files.)
But once you find it (or them), try renaming them to *.xlbOLD. Then start up
excel again and see if that helped.
If it did, then I'd delete those files. But if you have customized your
toolbars, you'll have to rebuild them.
After you've finished rebuilding that *.xlb file, find it and back it up.
If/when the real one gets corrupted again, you can just copy this nice fresh
backup to replace the "real" one.