Windows 7 Everything got delete

I installed windows 7 onto my windows xp coputer. The installation went fine(I think) but when I rebooted my computer everything was gone. My documents, programs, music, photos were all missing. I dont know how to retrieve all of my lost file, in which some were very important. If i re-install windows xp can I get all my old files back or is their a way to do so without going back to xp. I tried to do system recovery but all that showed up were the things that I have done since I install windows 7.
Thank you