AVG AntiVirus freeware by Grisoft.
Grisoft has a free for private individuals version of their anti-virus software, complete with automatic virus signature updates and realtime scanning protection. It has a low memory footprint so it won’t slow down your system while it’s running in the background. AVG Free has an easy to use interface so I often recommend it for computer novices. It doesn’t catch every virus, but honestly no single virus protection software package is foolproof these days.

Tip: It is possible to upgrade AVG 7.5 even if it is giving you a message to purchase their professional version. Just download AVG8 and install over the top of it.