Data lost


i have an internal wd640 and recently i got an xp problem after installing sp3. but after fixing i thought everything was ok. the next day i noticed that my drive was gone and when i tried to reinstall it i found out that it changed into a FAT drive of 10 mb (not gb) with only a directory of bitcomet on.

i am quite sure all my data must be still on there but how and with which software can i recover it ? thanks a lot