This is a tough question but this is how I do it.
1) Cost. Free is good.
2) Name. It is important that it is a known good antivirus.
3) Ease of use. I like a neat control panel and a useful help section
I use AVG, free version because of this, however I have used Norton before that. It cost £30 a year and updated automatically weekly but it hogged the resources and was quite invasive.
I also use Spybot and MalwareBytes Anti-malware for clearing up. You should also run Spybot's teatimer for realtime protection if you choose it. I like Spybot but it's been superceded by such things as MBAM and Superantispyware...
Finally, I have Hijackthis and Blacklight for rootkit detection.
All recommended software is in this forum thread