Shiny Store Bought Computer: look online, find the model on the manufacturers site, look for flash bios for that model of computer. D/L the proggie, follow the instructions, and pray.
But it could be two other things. One, the boot sector. In which case... clean install with Low level formatting, backing up, etc.
but another poster reminded me of a problem I had with a 2k box... and that was the APM (i think it is called that). Anyway, it is a service in the Computer Management section I belive called "Automated Power Management". Double click on that, choose disabled (if i remember right). Some other poster needs to fill in and verify the details of this last step.
But you'll need SP2 too anyway, regardless of anything else. Might as well use windows update to go and get it. But i kinda doubt that wil fix it unless SP2 fixes that APM bug somehow. Not for sure on that.