Personally, I use AVG Free on most computers, combined with Lavasoft Ad-Aware SE Personal, Spybot Search & Destroy, CCleaner 2.0+ and either COMODO Personal Firewall or PC Tools Firewall Plus. However, on my Macbook's Vista partition I recently got a free copy of AVG Internet Security 8.0 via TrialPay, which encompasses pretty much encompasses all my needs - I do keep Ad-aware/spybot just in case, though.