MS outlook stores messages, calendar events, schedules and other objects in the PST (Personal Storage Table) file. .pst is the data file format used by it. This is Personal Storage File which retain all the data of this e-mail client.
The error messages, indicative of corruption, can shake the mind of user. One of such error messages is stated below:

“Unable to open your default email folders. The file D:…path to file…Outlook.pst is not a personal folders file”

After you click on the ‘OK’ button of this error message, the application will shut down.

This is the case when your PST has got corrupted and needs PST repair. MS outlook avails you the inbuilt utility, scanpst.exe, for this part which resolves many of the PST corruption issues.

You can find out more about this tool at the following URL :
http://support.microsoft.com/default...b;en-us;287497

Good old Microsoft rather than fix the issue they provide a tool to recover your screwed data

Remember to backup your data before screwing with it more than Microsoft have