Have you recently installed any new software? It is possible it is bundled in there somewhere.
I am surprised CWShredder didn't help you, but another excellent program is PestPatrol. You need to buy it, but on the first sweep (even after CWShredder) It found a lot of unwanted things on my PC.
http://www.spyany.com/program/pestpatrol.htm
Also, what Anti Virus are you running? Is it up to date?
Lastly, watch what sites you visit. Some of the more "adult" sites hit you with all kinds of crap.
I run, PestPatrol once a week, run Spysweeper, MCafee Virus Scan and personal firewall and rarely have any problems.
Good Luck!