The two free AV's I'd recommend are Avast! free edition and Avira free edition (formerly AntiVir Personal). Avast! free is by far my current favorite of all the free AV's and is packed full of features and with the anti-virus signatures being updated multiple times a day automatically by default. It performs as well in many detection tests as the top premium AV's do. If you want the best premium/paid Antivirus software or a full-blown Antivirus AND internet security suite (firewall etc) then I'd recommend either VIPRE Internet Security or Kaspersky. VIPRE's scanning engine is amazingly fast and highly accurate, it doesn't bog down your computer even during a full scan of your entire hard drive. It does really well in the malware tests too and is the most resource-friendly AV you'll find that is in the top 3 Antivirus solutions.

Kaspersky has always been the top dog (along with Eset's NOD32) but I no longer recommend NOD32 or anything from Eset because lately they haven't been doing as well as they used to in the detection tests. In fact, Norton has made a comeback and is now in the top 5, most of the time in the top 3 for detection rate scoring and other misc tests. NOD32, however, is still a great AV, lightweight, and resource-friendly (not quite as lightweight and efficient as VIPRE though). I would never recommend Norton regardless of how well it's currently doing so I'd go with buying (if you ever plan to go premium) Kaspersky or VIPRE; Kaspersky doing as good or better in most tests but is pretty unforgiving on your system resources with deep heuristics scanning and other CPU cycle expensive core features enabled.