Please regard this thread regarding this problem.
To summarize, it seems like Spambayes is not the only cause for such problems, although uninstalling it fixed the problem for some people. Other people had a problem with ActiveSync for the PDA, cell phone etc.
Still others have reported that's rather the spam filter that comes with outlook that's causing it to hang, and once turned off the problem was solved.
Please also have a look at this web site that sells a product named Shutdown Addin for $4.99. I quote from this site:
Outlook is notorious for hanging
around in the background when a user
exits the application. The cause of
this is partly a random behavior of
Outlook itself and most often a user
installed addin that does not release
itself from Outlook correctly.
Typically, the solution is to
uninstall the offending addin.
However, if you really like an addin
and can't wait for the developer to
fix it or have no addin's installed
and Outlook STILL stays running the
background then you'll want to try