This is going to sound fucked up but I had the same thing happen to me (twice).
Now the first time this happened I fucked around with it for about a day using the windows 2000 boot disks to format because I was desperate and couldn't find anything else that worked....Well that worked half assed. Then about a month later the same thing happened and I found it was worth my time to just go buy a new hard drive and fresh install xp then transfer over my old files...sell old hd on ebay
Like I said, it sounds fucked up.