Personally I am a huge fan of Avast! Its free, is very reliable, and I have personally used it to remove viruses from many computers of the last few years. Its "Boot time scan"mode is extremely powerful. (AGAIN ITS FREE, but does have paid versions).
Microsoft Security Essentials are good too. Even if you are careful you should ALWAYS have virus scan software. I also getting Malwarebytes anti-malware and running that once every 2 weeks to remove any malware.