Virus stops Windows XP booting

It is an ongoing battle with Viruses. I seem to go for a month without finding any and then suddenly I find a stack of them. The latest is another version of the Fake Anti-Virus virus. It gets cleaned up with the usual tools. You do need to go into safe mode as it disables running any other program while in normal mode.
A new twist however with a computer I had to go and revisit a week after I had cleaned it. The computer would not boot. I tried a repair install without any luck either. In case it was hardware related I started it as a Virtual Machine on my main workshop computer and it had the same problem.
I was about to do a clean reinstall of Windows XP and noticed one of the NVIDIA drivers was just replaced. The HDD was slaved to my workshop computer and when I clicked on the driver NOD32 went crazy. The file was NVATA.SYS (involved with the HDD controllers). I found the original version and replaced the file and the computer restarted without a hitch. Strangely a complete scan of the computer failed to find any other viruses.