My pc sending spam

Hello,

I noticed slow internet performance on the PCs and MACs in my house. I discovered disconnecting the network cable on one PC solved the problem. I then purchased Norton AntiVirus 2010 (yesterday) and ran all the scans removing several viruses.

 

The PC still appears to be sending spam. When connected to my network it is really slow and un responsive. Once I disconnect the network cable I can use it again.

 

Also when connected I get a series of Noron popup messages: 

Norton AntiVirus Email Error 

Then there are messages such as "550 requested action not taken, "user not valid", "email address removed due to high levels of activity" etc.

Then the message details are 

From "Viagra (c) Best Online Store <random email address>

To <random email address>

Subject User fangsx special 80% off.

 

I have turned the WIndows XP firewall to on with no exceptions as well as run several Norton AnitVirus scans and I still continue to get these messages. Sometimes 10 or 20 in the 60 seconds the network cable is plugged in.

 

What is going on?

 

 

Thanks in advance!