You sure it's not a virus or something else and not Outlook Express?

Sorry, it doesn't take a virus for Outlook to eat its e-mail database. You can simply look at the thing sideways and it will die.

That's why I've been using Forte's Agent exclusively for as long as I've been on the internet. Rock-stable, and I've never ever lost an e-mail message. Let's see, the oldest message currently sitting in my Agent database is Feb 2000, and if I wasn't so anal about cleaning up old messages, I'd probably have stuff in there from 1996.