Was this originally an upgrade from Outlook 2002? There are sometimes issues whereby the .pst files exceed 2Gb or become corrupted.
To fix corrupted PST files, you can use the Inbox Repair Tool. However, the tool isn't always able to repair every PST file. It works by repairing the PST file's header and then deleting anything in the file that it doesn't understand. So if a PST file's header is damaged, as may be the case for corruption that occurs during a version upgrade, the tool should have no trouble making the repair. But if the data within the file is corrupt, the Inbox Repair Tool will likely destroy what's left of the file. That's why it's always good to make a backup of the PST file before running the Inbox Repair Tool.
The Inbox Repair Tool is located in the C:\Program Files\Common Files\System\Mapi\1033\NT directory of any system that's running Windows 2000 or Windows XP. The tool's filename is Scanpst.exe.
There is another tool from Microsoft that can be used if the pst file has exceeded the 2Gb limit is Oversized PST And OST Crop Utility which you can use first, and then follow up with the Inbox Repair Tool. The crop utility works by deleting about 25 MB of data from the file to bring the file back within specs. Use this tool as a last resort, because you can't control what data you lose when using it.
This is available for download here: http://support.microsoft.com/default...;en-us;Q296088
Lastly, there are other utilities out there from other companies such as Filechopper (available on ZDNet), etc