I've been trying out Microsoft Security Essentials on one of my computers at home. Everything had been going fine. It took care of spyware/adware as well as any other free scanners out there and I noticed my computer operated faster with a single unified product as opposed to two separate products (I have found over the years that in a home situation, the slight increase in protection/performance over free products is not worth the expense of paying for AV/spyware protection).
Then, I get a virus. Security Essentials failed to notice a 5 year old Vundo variation one of my kids downloaded and caused an annoying infection that I ended up just backing up my files and wiping XP and starting over (after 3 days of doing it the right way and still coming across re-infections and rogue registry entries, it took me less time to just reinstall everything).
Of course, AVG free picked it up on my other computer and eliminated it with no issues.
So, the gist is, don't bother with Microsoft Security Essentials.