2000, I have no idea. However, there is no difference between 2002 and 2003 (in my opinion). The features seem to be just the same. I personally do not like 2003's email scanning function. Perhaps I am just used to 2002's features. I have used both but have reverted back to 2002. Hope this helps.
the main difference from 2002 is that it can prevent you from downloading viruses in instant messengers such as yahoo,msn,aol etc.i think its pretty pointless.dunno about 2000,but if you have 2000,upgrading would probably be a good idea