Try using SCANPST.exe utilty which should be within your Outlook directory someplace. If not it's on the Office CD.
If SCANPST fatals out, then you've probably got a virus embedded in one of your messages. To resolve, go into your antivirus program and have it scan compressed files down to 10 levels. If still no go, then you'll have to resort to a third-party PST fix util. There are a couple out on the net.
Good luck!