If it's the OS (If it's not one of the things mage pointed out which are common problems I have seen with your problem as well)

If you can if the computer starts into windows you can access your Recovery Console in win2k by Start ->Programs ->System Tools -> Back-up. I would recommend you make a copy of the files located in \system32\Config so if anything happens you can restore your Registry.

If you can't and If you still have your windows 2000 disk I would say boot from that (I'm going to hope you know how to do this as for me to explain this step may depend on your Bios) and press "R" at the Welcome to set-up screen. Then select "R" again when prompted to choose the ERP (emergency repair process) This will give you the last prompt in which you select "F" for fast repair. This repairs the MBR (Master boot record) as well alot of other things about the OS. Remember this DOES NOT repair appilcation, your data, or hardware problems.