If a clean install of Vista fails we start looking for hardware issues such as some USB device plugged in, etc.
After vista installed an update, the computer would not start up anymore. After the vista loading screen, it would just go black and stay like that. I tried installing vista a few times (clean installation, booting from the cd), but it would get stuck on the "completing installation" screen, even when left on overnight. Installing xp was also a failure because it did not recognize the space on the hard drive, partitioned or not (but the vista installation did).