None, most antivirus "products" are as bad or worse than viruses. They make changes to your computer without your permission, they hog system resources when you are trying to do something else, and they prevent legitimate operations because they can't be configured properly.
Use firefox w/ adblock (ad servers are a major infection vector for browser exploits), don't run suspicious executables, and if you are paranoid occasionally run wireshark to sniff the traffic coming out of your machine during idle states.
Modern viruses try to add you to botnets so they are usually very polite to avoid detection.