About a month ago he was getting frequent popups claiming that there were disk errors, but I think that was some malware. I cleaned out a bunch of stuff (mainly using Spybot S+D) and after that it seemed to be fine. I did a chkdsk which did not report any disk problems. At that time I also copied all of his user files onto an external USB drive.
Apart from that, I don't know much about what led up to the current problem. I am sure he would not have installed new hardware or mucked around with system settings.
I have run all of the diagnostics from the Drivers & Utilities disk and the hardware seems completely fine.
I have used the AVG rescue CD to do a virus scan...it found a few and deleted them.
I have used the "repair startup problems" function on the Vista CD and it said there were no problems found.
But the system still does not start up.
The only thing I can think of to do next is completely re-install the OS, but I would like to avoid that if possible. Does anyone have suggestions for something less radical?