I have said it many times but noone believes, MSE is one of the worst antivirus programs out there! Yes it is light on resources but it sucks big time in protection!
After perusing the AV-test site, I will still use MSE for these reasons.
1) Light on resources.
2) Free.
3) Good enough.
You of course will have an issue with 3. I do not. Why? Well, it scores well in the more prevalent malware at 100%, making it the equal of everything else on the market. What it scores poorly in is "0-day attacks inclusive of web and e-mail threats." Really? People still get infected through clicking on attachments? I'll take that risk.
In fact, I didn't run AV until I started using MSE for Win7. I've only been infected twice in... ever. First time, someone else was using my computer when I was away and browsed to some iffy site. Second time an infected PDF was loaded through an ad on a tech site (not AnandTech, but probably well known enough) and the notebook didn't have an updated Adobe Reader. That notebook had not been used in well over a year and Reader was probably 2 versions old by then. Thus, I consider both those instances to have been avoidable. Oh yeah, that person who got my computer infected was always getting his own computer infected too.