Well, one of the 3 different types of 'hard drive' that has failed has been a proper external one, with it's own power cord which plugs into the mains supply, so it shouldn't be drawing too much power from the computers PSU. I've had a look at the connections on the motherboard, and to be honest, if there was a problem, I wouldn't really know. Everything seems to be ok, nothing looks damaged or burnt or anything like that. If i could just narrow it down to one or two pieces of hardware, i'm happy to replace it. But to find out exactly what's wrong, it seems like I'll have to pay a bundle just for a diagnosis from somewhere, and then pay even more to fix the problem.

Thanks for the continuing responses.

I'm trying to get hold of an SP2 CD to try that check.