Well I don't know what caused it but i think I've found the problem, in case it rings any bells with anyone.....
About 20 minutes after posting the above, the exact same thing happened again. I tried to write to the upper part of the 160GB drive (via Windows 2000) and then discovered the entire partition table wiped...!
I then realised that whilst Windows XP was correctly reporting the drive (i.e. the physical drive) as being 157GB, Windows 2000 seemed to think it was only 131GB. I've now wiped the entire drive and recreated the partitions. At the moment, both OS's agree that the drive is 157GB, so I'm hoping for the best...
I reckon this must be connected somehow to my motherboard upgrade because I'm certain I would have noticed this problem before now. Why exactly it's happened though is a total mystery
