Pretty good, does the job well, Uses only 25MB at idle and 50-60MB while running a scan. im using an ongoing beta, a bit more new version than the public release, but can recommend MSE, and NOD32 also...
MSE uses the AV Engine of a high end System(Forefront) so it should be good.
and also the best part is its totally FREE, if you have a Genuine Windows(not sure whether it works in cracked ones though) you can install it easily, installs in <1 min. no need to restart...
The only poor thing in this is that this doesnt have any Internet Security, so i have to use Comodo Firewall..