First, go into control panel and remove any software that may have been added to your computer against your wishes.
Next, download ccleaner @
www.ccleaner.com, and use it to scrub out your internet left-overs. Then do the usual AdAware SE, Spybot S&D, XClean (at spywareinfo.com), clean up efforts. Then run a registry cleaner product like registry mechanic to clean up unused .dlls, extensions, etc. Do these on a regular basis. Run spywareblaster in the background. Set your browser settings against re-directs, auto launches, autodownloads, etc without explicit permission.
Then run whatever anti-virus you want, because you just cut 90% of it's workload out by sensible surfing and prevention measures.
And +3 on NAV is crap, but that's just my opinion.