The last time I installed MSE on a bunch of office PCs running WinXP, it rendered them completely unusable... memory usage directly after a cold boot jumped up to around ~650MB, which was rather sad as they only had 512MB physical memory apiece. Prior to installing MSE they'd been hovering around the 150-200MB mark, and even uninstalling MSE didn't fix things, I ended up having to restore from previous system images to get memory usage back to sane levels.
I'm not sure if it was a bug which has since been fixed, but I don't feel inclined to try again, particularly as MSE doesn't seem to be a particularly strong performer these days in any case:
http://www.av-test.org/certifications?order=protection_desc&lang=en
I tried Avira and found it to be a dog, its definition updates were hopelessly unreliable and it had an enormous, silly popup - yes, there are ways to kill the latter, but you shouldn't have to.
So I'd go with Avast out of the free AVs, but only on the basis that it's the best of a bad lot - they're all horrible IMHO, and only for use on someone else's PC where you know they'll be doing "stuff" which you're unable to continuously monitor and supervise, and some protection is better than nothing in that situation.