I had the same problem until recently something odd happened. I was using norton partition magic making new partitions and I have a 2nd hard drive i installed linux on that then my missing gigs appeared i formatted them and now i have it all back. I dont no why it happen though freak occurance probably :S. Or it could have just been me messing around with my partitions that forced my missing gigs too appear.
I also reformatted using xpsp2 and did it twice! and still couldnt get my missing gigs back until what happened above ^.