I never find problems on my PC?

I've got Avast! for anti-virus protection, I use Spybot S&D and MalwareBytes to keep spyware, ad-ware, malware, etc. down. I use Zonealarm for a firewall. I keep them all up-to-date to their current versions, but everytime I scan with any of them they never find anything. I'm a pretty computer savvy person so I know how to keep garbage to a minimal pretty much, but still I would think something would get through eventually. It's been like this for a year or two now. They'll find the typical tracking cookies and all of that. Rarely Avast! will find something trying to come along that it blocks and I just get rid of it then. Do you think my PC is that well protected? Or maybe I need to get different protection? I've tried Ad-Aware out and it never finds anything either. I don't download very much stuff and I don't visit highly unsafe websites. Maybe there isn't anything on my PC, but I'd like to make sure! I don't want anything on here that'd be disabling everything. I've edited my boot.ini so that my PC only starts up with what I feel is necessary. I've looked in my process tree before too and I never see anything out of the ordinary running in my background. Just post some replies, add in what anti-virus software and other software you use to keep garbage out of your computers.