BitDefender Free Edition
BitDefender is a first-class product with excellent detection rates. Everyone knows that it is very important to have your PC protected while surfing the internet and using programs borrowed from your friends.
If you are habitually downloading programs and joining P2P networks, you better be protected and bitedefender is really a good choice.
It will protect your system against numerous viruses, spyware, trojans, adware,... It doesn't affect the performance of your system, so you can install it and won't even notice it is running. Update it daily and keep your PC safe.
It is easy to use, just run it and let it erase the malware that may affect your PC. Don't hesitate, Bitdefender is one of the top antivirus you can find and it is completely free.