The computer is a Dell Dimension 4100 running Windows XP. The lady opened an email and the computer crashed and started continuously rebooting.
First the computer would boot only as far as the Windows splash screen, then reboot.
So I tried chkdsk, fixboot, fixmbr--didn't work.
I tried Ultimate boot disk--but the restore wizard didn't fix the problem.
Then I thought the email thing might just be a coincidence (and there were two orange lights and two green lights on the back), so this might be a hardware/memory problem. I took out all nonessential PCI cards, switched RAM, etc., but nothing helped. Then I tried another hard drive (from a different 4100), and I was able to boot into Windows with the different drive.
So now I'm thinking my first thought was right: a boot virus.
I also tried booting with a Windows 98 startup disk, doing fdisk /mbr, then in Recovery Console, fixboot and fix mbr. Didn't work.
But now the computer gets past the Windows splash screen before it reboots.
I tried slaving the drive in another computer and scanning for viruses, and it just found negligible stuff. Can read all the files okay.
I'm running out of ideas. Any suggestions for what to try next?