Only supports Windows XP and Vista. You might want to just renew the subscription of the current Norton that is installed in Windows 2000.
Or get another:
Below are free antivirus that you might want to try. It supports Windows 2000:
AVG Free - http://free.grisoft.com/doc/5390/lng/us/tpl/v5
Antivir (free) - http://www.free-av.com
Avast Home (Free) - http://www.avast.com/eng/avast_4_home.html