Tried the freeware route. Got bitten once.
Learned a bit, tried some different stuff, experimented heaps, read lots of forums.
Got it right, now. Malware free for 3 years, round about.
One of the more effective things you can do to prevent "drive by downloads" is to install the NoScript add-on for Firefox. Hidden malware code buried in a (usually hacked) web page just can not download nor install. Doesn't take too long to get the hang of using, and will also very nicely block some ads and banners, thus allowing pages to load faster. Adblock Plus is another good add-on.
I use Avast (free) 5. (And 4.8 before that.) I haven't had a virus get through while using it. No antivirus is foolproof, though. All of them (Avast included; Kaspersky included) should be treated as though a virus might not be detected by them. Which basically means using NoScript, block lists (below) and not clicking on any old thing or attachment, nor trusting any USB drive someone has loaned you.
MVPS Hosts file is a useful block list to install, and an application called Hostsman to easily manage/update it. This will block known bad URL's. It needs to be updated periodically, about once a week. SpywareBlaster, by Javacool, achieves a similar result.
Keeping all software up to date is important, including Java and Flash players.
MBAM (MalwareBytes anti malware) is an excellent demand scanner that will provide a good second opinion for scans, plus very good removal abilities. It works in a similar manner to old Spybot (which is still supported, BTW) and there is a free version.
All the above are free, or have a free version. Minimal system impact. Reliable.