I used Avast until I tried "ThreatFire" by PC Tools. ThreatFire is behavior based. It looks for the behavior patterns of malware; viruses; trojans, etc. Then it eats them. It just works very quietly on Windows toolbar. You don't have to do scans or anything. It does it all for you. I have NOT had any problems since I began using it a few years ago. I have a paid subscription, but you can try a free edition. Don't use any other anti-virus programs with it!
ThreatFire Behavior-Based Anti-Virus and Spyware Protection Free Download