I don't know if Outlook Express uses pst files the same as regular Outlook or not. If so, you might try replacing your current version(s) of the pst file(s) with those from your most recent backup (you are backing up, right...). I have found that as these files swell to large size (and they can get very big if you're filing/archiving emails as opposed to just deleting them when done) and are then prone to becoming corrupted.
This is what happened on my XP machines, running the 2003 version of Outlook. So far, a month or so into my Win-7/Office-07 journey, no such problems.
Perhaps it's a timebomb built into the software so as to force you to buy new hardware and of course, software.