I've tried some of the other "free" antivirus programs that others have listed here, and they all seem to slo my computer down. Here is what I have found:
AVG - This is popular for the free version, but after 30 days you've got to buy it. Again with this one, it was slow. I also didn't see any benefits of buying the paid version.
PREVX - Good free scanner, but they are only a scanner - you need to buy the software to remove anything. They have a very cartoony look to it.
PC Tools - It does ahve a nice interface but since they are a private company, who knows how stable they are, but their software does work for free.
Norton 360 - Expensive. My experience with NOrton was not very good and it was very slow. They are a NASDAQ company.
Cyberdefender - Great scanner removes Trojans, spyware, but you do need to do the upgrade to remove viruses. They do offer 24/7 computer help on email or vy phone. Plus, they are also a NASDAQ company.
When I looked at and weighed the pros and cons, I found that Cyberdefender was the right choice for me.
I wish you the best of luck in your search and I hope this helps.