If you have an older Windows computer you may want to take a look at Panda Cloud AntiVirus. It consists of a lightweight antivirus agent that is connected in real-time to PandaLabs' online Collective servers to protect faster against the newest malware variants. Since, it does not have to download virus signature updates (it uses a database in the cloud) it is very light and uses little memory and cpu. It is free for home use and is very effective in preventing malware infestation.