The HD is not likely given the symptoms. Slow disk access is because it's thrashing like crazy over N360 mucking about and Vista's file indexing and reordering. The rest, BSODs, freezing, failure to boot or POST, all point to a physical memory problem. Because he's using the same MB and memory as when it ran just fine, I'm going with insufficient power and possibly the video card.
And I'm not a betting man - Especially when it's over something a frustrating as computer problems. Those are painful enough without a bunch of yahoos gambling about the outcome.