Have you tried clearing any temp files?

Xp:
Close out of outlook & any other open office programs.

I would try changing the name on the "outlook" and the "office" folder in your local settings

C:\Documents and Settings\<user>\Local Settings\Application Data\Microsoft

to "outlook.old" and "office.old" and see if that resolves the issue.

(the reason i don't just say delete them is in case it changes something that you may not want changed you can just remove the new ".old" tag)

If that doesn't fix it and you're confident in your ability to reconfigure your profile I would probably try rebuilding the mail profile.

control panel -> mail -> profiles

note: only do that if you know where your pst's are located and you're confident in your ability to reconfigure exchange, pop, or whatever you're using for mail.