You may have not gotten rid of your virus, did you reformat your drive before reinstalling xp? If not the virus may have reinstalled itself.
The other possibility, that when you went out on the Internet you picked up a new infection, is possible but less likely.
If you want to make sure that most updates are included when you reinstall, one option is to create a windows xp sp2 disk via slipstreaming. It is a little bit of work on the front end, but makes life a lot simpler for reinstallation and repair tasks. Particularly if you do this more than once.
To create a windows xp sp2 disk you need a cd burner, a windows xp disk (not a system restore disk) and software that can create a bootable cd. Of course you also need a computer that is functioning.
For complete instructions see:http://www.winsupersite.com/showcase/windo..._slipstream.asp
There are a number of steps in the process, but it's not quite as bad as it looks. Also if it doesn't work the first time (it took me 3 tries) you are out a little time and a couple of cds. Nothing is changed in your system.