You probably got Blaster or Sasser or some variant. They'll infect you if you use an outdated version of 2000 or XP, with all the vulnerable services left running.
You should reformat, and install 2000 again and this time without your internet cable attached.
Then run
this program (of course you'll have to download it beforehand and save it to a floppy-disk or something) from the command line* with the /all option. This disables all services that could be attacked, giving you time to connect to the internet and download all service packs and patches.
* go to start --> run, type in cmd and press enter.
From there you need to navigate to where you saved the svc2kxp.cmd file with the cd (change directory) command. cd.. goes a directory down. If you've saved it to a diskette you get there by typing a:
You run the program by typing