Personally, I would not recommend Microsoft Security Essentials(MSE) only because there are other free AV packages that have clearly tested superior in protection by independent security groups. I suppose MSE is better than nothing, though even Microsoft recommends using third party protection, conceding that MSE should be considered basic protection. They just haven't put the tech resources into MSE to make it at least equivalent to many currently available Free AV packages.
One of the current highest rated Free AV packages is
Comodo Internet Security. Even higher than Symantec Norton Internet Security in protection. While having having a significantly lower effect on system performance. One of the best rated of any paid or free AV package.
Avast Free AntiVirus and
AVG AntiVirus Free 2014 are both excellent free choices, though tested slightly lower in protection than the paid Norton. But significantly higher in protection than MSE.
All three of the above Free AV packages support Windows XP/Vista/7
Source:
AV-Test