Mine has my email sending out spam to everybody in the known universe and I am getting constant cannot deliver messages bacvk. 5000 yesterday and another 4000 (so far) today.
A firewall may help you contain the problem (though not cure it). Remember, the firewall included with Windows XP is only half a firewall; it guards against incoming attacks, but does not allow the user to control/monitor outgoing traffic. (See my FR homepage for links to several freeware firewalls.)
Norton scanning now. AVG scanned yesterday and AntiVir sacan waiting.
You may also want to scan with
Antidote SuperLite. Though Antidote doesn't clean (scans only) it has a very large database, so not much gets by it.
If you haven't tried scanning with these, you probably should:
Ad-Aware SESpybot - Search and Destroy