Yes that is where all you outlook data is stored, you are not seeing the actual extension because you are hideing extensions of known file types, uncheck that box under tools, folder options view and you will see that it says Outlook.pst on another note that's a pretty large .pst file earlier versions of Outlook, seemed to have some troubles if your .pst file started getting close to 2 gigs, yours is 1.6 gigs, I believe that Outlook 2007 handles larger .pst files pretty well so if you have outlook 2007 installed or are upgrading to it, kindly disregard. But from a shear performance standpoint you might think about archiving and compacting. Just a thought