Something that is being opened is causing trouble.
It could be an addin checked in Tools|Addins or a file in your XLStart folder
(or if you are using tools|options|General|"Alternate Startup File location", it
could be in there.)
(Not many use the alternate startup file location, so try erasing that value
from that box in excel--don't delete the real folder. If that fixed it, you're
done.)
If not, your task will be to uncheck everything in tools|addins, remove all the
files from the XLStart (and alternate startup location) and start adding things
back one by one.
And restart excel between each change--until you find the culprit.
Chip Pearson has some additional notes to help diagnose this kind of problem at:
http://www.cpearson.com/excel/StartupErrors.htm
