I've used a few different AntiVirus/AntiSpyware software..
1. Norton
2. McAfee
3. AVG
4. Avast
5. PC Tools
6. ESET (NOD32)
7. Trend Micro
8. Kaspersky
9. SpyBot Search & Destroy
Category A: (1,2,3,4)
- Never catches anything throughout your entire subscription
- Crap interface
- Heavy on system resources/slows down everything else
Category B: (6,7,8,9)
- Catches everything throughout your entire subscription
- Good/Simple/Standard interface
- Tons of false positives
- Medium on system resources/can slow things down
- System Tray Pop-ups that bug you much more often than Vista's User Account Control
Category C: (5)
- Catches everything throughout your entire subscription
- Good/Simple/Standard interface
- Minimal false positives
- Light on system resources, no noticeable performance impact
- Although there is occasional pop-ups, they are only what is absolutely necessary.
- Additional features such as "Game Mode"
Essentially, I would highly recommend PC Tools Spyware Doctor with AntiVirus (2010 Version). All the other AntiVirus software I've used fell into one of two "bad" categories, but this one did not. Highly exceeded my expectations and is well worth the $40. Just trying it out wouldn't be a bad idea. They have a trial, and if you do decide to purchase there is also a 30 day money back guarantee. PC Tools support has been helpful and nice in the past as well. When switching to Windows 7 the software I had a subscription for was incompatible. They were able to switch me to a newer version that was compatible free of charge.
If you can not afford such though, then I would suggest that if you could identify problem files on your own then you could upload it to:
http://www.virustotal.com/