Okay, sounds like it's corrupt to me. There's a few things you can do at this point, and it's what I'd do, and in the order I'd do it:
Run a repair on your PST file:
(
http://www.slipstick.com/problems/scanpst.htm)
If that doesn't work, create a new PST file. Grab each of your current folders that contain anything and copy them into your new PST file. (File-New, Personal folder, blah, blah.) Get everything in there that you possibly can, doesn't matter what order, but just put them in folder names you'll recognize and all in one folder. Then Export that file to a PST. (File-Import and Export, to a personal folder file, blah, blah, put it somewhere you remember, call it Outlook.pst)
Close Outlook, find and rename the Outlook.pst that outlook is currently using (it WON'T be the one you just created, so don't use that one) and rename it to OldOutlook.pst. Put your new outlook.pst in the same folder as OldOutlook.pst. Open Outlook and it should have your folders in there. Drag all the messages and stuff into the appropriate folders again.
Sounds like a lot to do, but that's because I tend to run off at the mouth. It's not much, really.