McAfee says no virus, but have symptoms

I swear I have a virus or spyware on my home PC. I get spam mail that uses words I've searched for or have in my address book, that kind of thing. But I've run McAfee and it says the system is clean.

I've also run Ad-Aware and quarantined anything it's found, but it hasn't shown anything really dangerous. And the behavior continues.

I didn't want to post any Hijack This log unless asked. I haven't scanned with it yet because I didn't know if it was necessary.

Should I trust the McAfee results?