Do you have a wireless router? If so, check that you have wireless security enabled (preferably WPA); you may also want to change your wireless password just in case.
I'd also suggest using other malware programs, specifically SpyBot Search & Destroy and possibly Avast or AVG Anti-Virus (all free for personal use).
If you're really paranoid, get a live Linux CD (e.g. Ubuntu), boot off that, and use ClamAV to scan your hard drive. More work, but it might catch hidden ("root kit") malware.
Your firewall should also be able to tell you what is generating traffic.
Finally, presuming you have a cable or DSL modem with blinken lights, you can watch them for activity.
Worst case: take your computer to trusted shop (if you can find one) to be scanned. They can scan the HD without booting from it, and should be using multiple good quality tools. (A hack shop, though, will just run basically what you've done.)
ETA: The reason for the wireless question is that there are at least two attack vectors. One is via your wireless network, and would have nothing to do with your computer; it'd be basically someone nearby connecting to your network. The other attack vector - and the one you're suspecting - is malware on your computer(s). [If you have multiple computers, or other networked devices, they should be checked as well.]