I agree whole-heartedly with OldBuddy... Even the best AV programs aren't completely virus-resistant. For example, I used to have a Windows XP system that was running McAffee's best AV system (AV program as well as a firewall and other security software) that caught a nasty virus. While I was working with Tech support for getting rid of the virus, I was told that the reason why it passed my AV protection was because it was a brand new virus that the AV software wasn't able to recognize. I guess I was actually working with their new malware department to figure out how to detect it as well as how to fix it. That's why with even the best AV program you have to continually get new updates; there are always new viruses coming out and some might not even be recognized by the AV software yet!
Moving to a linux based system is a little better since most viruses are made to target Windows and because the older Windows OS's allowed programs to be installed without getting permission from the computer's administrator (starting with Vista, Microsoft is starting to clamp down on what programs can be installed, although it still is not completely safe).
I would NEVER trust some AV program that you find online (unless it is coming from a major AV software company). There are actually several trojans that run like the mafia's protection racket scheme: install their "AV software" for free (which is actually the trojan), run the scan to "see" all of the infections (which it will claim there are hundreds that have infested your system when in fact only that program has now infected your computer), and pay money to "remove" these viruses and keep your computer safe, whereby they mean they will inactivate their trojan for a year if you pay them for "protection" but will activate the trojan if you decide not to pay (see here:
http://www.bleepingcomputer.com/virus-removal/remove-antivirus-vista-2010). My dad accidentally got his laptop infected with this by being sucked in by some popup alert that showed all of the "viruses" his laptop was infected with. When he clicked on what he thought was his own AV program, it actually downloaded the trojan and then gave him the option to pay to remove all of the viruses. Since he had already paid for his other AV program, he got suspicious and called me. I was able to research this trojan and remove it without it damaging other files or him paying anything to this fake AV company.