Some trojans don't use your contacts. They often just open up ports to allow your pc to relay their email. So they won't be traceble when people complain. All the fingers will point at you. Usually if there is a high activity of mail, your ISP will send you an email and advise you, they usually give you a few days to sort out, then threaten to cut you off till problem has been resolved.
There are trojans out there, that stop these antispyware and anti virus from picking them up. They do this by setting up exclutions in your anti-virus/anti-spy tables. Might be an idea to check your exclutions. Also check your exclutions on the MS firewall.
Got to McAfee's website and download the latest STINGER.EXE. Its free. It is a stand alone program, that when excecuted will do a scan of your system and check for all sorts of crap and tell you about it, might not be able to fix it, but at least you know about it. (It should not install the program, just run when executed, they advise people to run it before installing one of their products)