Latest tests actually show MSE having pretty horrible protection, how ever if it makes you feel "safe" then sure go for it. You're probably just as safe as not having AV + normal caution.
ZoneAlarm Free and AVG free are some of the better free ones out at the moment.
Personally I find it funny when people spend $1000-2000+ on their PC's and they won't spend $30-50 a year to stay safer, I'm talking about full protection not just AV. But to each his own, I'm not effected by it.
Oh and way too many "I use this so it's good...", I bet many of those using the free less than decent options wouldn't have gotten any viruses either way.