Well it looks like you have an undetected trojan spambot (which has its own small SMTP program) at work.

What is your firewall ?

The spambot may well be hidden by a rootkit, try there anti-rootkit tools:

Also see, anti-rootkit, detection, removal & protection http://www.antirootkit.com/software/index.htm.

If you haven’t already got this software (freeware), download, install, update and run it, preferably in safe mode after using one or moe of the anti-rootkit tools.
AVG anti-spyware (formerly Ewido). Or SUPERantispyware Or Spyware Terminator.