I turn my computer on and just before it starts to show my desktop it shuts down and re-starts all over again. It's in a continuous loop of starting up and shutting down

First, open machine up and dust it. Then let it cool off for a while. Restart, and if there still a problem, right before you see that pretty Windows 2000 Professional screed with the blue squares that grow, it says press F8... Try that and see if you can boot in safe mode, or command propmt. If you can do that, backup any data that you want to protect. If you boot to safe mode, run disc defrag Start-Programs-Accessories-SystemTools, then try D:\i386\setup.exe and repair windows installation. If command prompt D:\bootdisk\makeboot a: You will need 4 blank discs for this. Boot to the disc, and repair windows this way. Hope this helps!

I've seen this type of behavior before. In my case it was a flakey power supply. (Have also seen a similar problem when a CPU fan had gone out and once the processor got too hot the system would shutdown or crash).