Shut down the computer, pull the drive out. Every moment the computer is running there's a chance that more and more of the files will become irrecoverable as stuff gets written over them.
After that, download a program like Recuva (by Piriform, google it up, I'm sure you'll find it), plug the drive in an another computer and then just use the programs to find&recover the files (whichever ones it can) - but DO NOT SAVE THE FILES ON THE DRIVE YOU'RE RECOVERING THEM FROM.
Also, there's no guarantee that all -or any- of the files will be usable. Reinstall is a pretty big operation, lotsa stuff might've gotten written over. But it's worth a shot.