Thanks for the replys!
I got the vista install from the link Willy provided, and ran the X14-63453.exe to make the Vista folder, used IMGBurn to turn the Vista folder into an .iso, and burned it at 8x speed to a blank DVD, which I made bootable. The computer didn't come with recovery disks, but had a recover partion on the failed hard drive.
I tried the install with the old HD unplugged, and got the same result. I will have to try switching out the SATA cables in the PC so that the old HD's sata cable is attached to the new HD. I've tried the formatting and erasing within the windows installation, but I'll try DBAN or KILLDISK to do a full HD wipe. If not I will probably have to consider Robert's advice and try replacing the CPU heatsink, though to be honest that sounds like a lot of work.
Hopefully one of these things will fix the problem. Thanks so much for both your help.