I use AVG (grisoft.com) Free but we have registered option at work. I also use spybot and adaware (free spyware) to check once a week.

I use all the above with good success. I also use crap cleaner (ccleaner) to "clean" the hard drive -- similar to scan disk but much faster and more thorough.

All the above can be downloaded free at filehippo.

And since were discussing it, I also use registry mechanic. It costs $30/yr. but after using it, and all the above, my machine went from a slow pig to a screaming fast machine.

YMMV