did you wipe your hard drive and reinstall from scratch? if you didn't, do that. once a machine has been compromized that's the only way to be sure you got everything.
are you using windows? If so, restart the computer, connect to the internet, but don't open any browser, or anything. Open the command line, and type "netstat". That command should give you a list of your open connections. If there is a hidden routine sending spam, it will be listed there.
Of course, it could have been that you are using a Wi-fi connection, and someone else has been hijacking it. Happens all the time. If possible, use only wires.
Btw, if you are going to de a clean install, do a clean install, don't use one of those manipulated windows that claim to have it all; Most of the time they only have junk in them, and I've seen some of them being already infected with shit out of the box.
I would try using a program like Spybot Search and Destroy. Most of the current A/V programs tend to discriminate, if it is a "VIRUS" then they have the signature for it, if it is "SPYWARE/MALWARE" many of them don't unless you buy their deluxe type packages.
www.safer-networking.org is where you would get Spybot from, it's a very good application, and free.
possibly a rootkit on your pc, eg. botted. there is actually no 100% foolproof way to make it clean again except complete format. but even then it still might be there, but this is rare.
so format and reinstall