Norton. No wait, hear me out. As many here know, Norton has always been a pig of an antivirus. In 2007 and 2008, it was really, really bad. It went from just slowing computers down to making them unusable. In the face of consumer backlash - even non-technical users were boycotting it - they made Norton Antivirus 2009. It uses ~10MB of RAM, and is now one of the faster scanners out there. It still gets Advanced/Advanced+ ratings from AV Comparatives, and updates signatures about every 5 minutes. Plus it has a "gaming" mode which puts even less stress on your system.