Well, it looks like I'm installing Vista again. I got up this morning and the BIOS (I think) moaned something about a RAID (There is no RAID on this PC...), then when I rebooted it said "DISK BOOT FAILURE", then I rebooted again and it said "NTLDR is missing". I just tried the Vista repair option, and it says that there's no install of Vista on the machine, and the C drive seems to have lost it's drive label.
So, I think the HDD is dying. I'm just reinstalling anyway to see how long it lasts. There's nothing important on that drive except for Vista itself anyway.
EDIT: At least this time it doesn't think the PC has 16GB of RAM in it...
EDIT #2: Ok, after installing SP1, the PC has magically gained 14GB of RAM. I don't know if this is a good thing or if I should be frightened [smile]