Why isn't my anti-virus software catching this problem?

I've been running Malware Bytes and Spybot, but they insist there are no bad guys on my machine. That's obviously not true, because A) my computer is running insufferably slowly, always a sure sign of infection, and B) Spybot isn't catching anything at all, not even the harmless cookies you inevitably pick up when you surf around. I know I've got something, but it's flying under the radar of my two favorite programs. Suggestions?