Outlook.PST is corrupted.

1. Search: SCANPST.EXE

Microsoft provides MS Outlook with a tool named ScanPst which is capable of repairing somewhat damaged .pst files. It scans and attempts to repair a specified .PST file; it analyzes a .PST file directory structure and checks mail item references and headers. After completing the scanning, ScanPst will display information on errors detected and offer you to repair the .PST file.

Normally, it stays in
c:\Program Files\Common files\System\MSMAPI\1033
or at
c:\Program Files\Microsoft Office\Office12

2. Locate the .PST file that needs repairing.

Resources: http://support.microsoft.com/kb/272227