mine is located in:

C:\Documents and Settings\Administrator\Local Settings\Application Data\Microsoft\Outlook

tho yours would be different based on the specific username (i'm talking windows username here). Administrator would be something like John, or BillBob.

tho, it is possible to change the folder that outlook looks to, and saves/loads from. i do know that it defaults to hidden folders. so if you manually search, make sure hidden files are being shown. or just search for all .pst files.