MSE is a great anti-virus program for PC's. If the OP gets a PC, get that to protect your computer....
The free Microsoft Security Essentials is actually a pretty good AV program and uses minimal system resources. That and most newer PCs have multiple cores, so the AV impact is pretty minimal. Any computer should have AV software as well as firewall software unless you either never connect to the internet or never load anything from a disc or USB stick. It's only prudent. That and some common sense when surfing or opening files/emails.