On the Wifes PC loaded on Microsoft security essentials a while back and have to say its worked well found Viruses that avira missed. The scan takes a while but it integrates nicely of course with Windows and sits quietly in the background. Scans take a while but have been effective for what it is.
Have a read of the CNET reviews
Not perfect but does a decent job and with little to fiddle with and used with Malwares spybot, or your choice of anti malware just sits there and deals with most things
Scans are slow though..
