I wonder if you use any instant messenger systems? MSN, Yahoo, AIM etc. Things might well get in through there.
Another fave, is a file sharing proggy like Kazaa, or Bearshare for instance.
I use Adaware and the free MS anti spyware program to attempt to keep things as clean as possible, but once your email address is in the public domain, then I am afraid, to use a technical term - you are stuffed.
Conan