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.

Avast anti virus freeware by ALWIL Software.
This program almost always finds infection that the other scanners miss. ALWIL makes their Avast! virus protection package free for personal use. I’ve found some tough to find trojans with this thing. I especially like it’s scan on boot function. It has a single excutable scanner that works fast, and appears to be a full featured protection package.