In the numerous cases that I've seen this, it's resolved by clearing the Outlook offline cache folder (or outlook temporary cache folder, whatever it's called).
I don't have the exact instructions with me now, but if you Google for 'clearing Outlook temporary folder' you should get some leads. The process basically involves you looking in the registry for the particular logged on user's profile to retrieve the location of the 'secure' folder (that was MS calls it) and then deleting the contents.
