You don't trust them to make an AV but you use their OS. 
They bought an AV company. What makes other companies more credible at making an AV than MS? Let's be rational here, we are technical minded people, we deal with facts. MS has everything to gain from having a good free AV, and it's been tested by independent studies with good marks.
Anyway, what matters is the what the tests like ICSA Labs, added to that the unobtrusive GUI and the fact that you don't even notice it's there, which is what an AV should be like, only pop up when there's a problem.
I have used AVG and I don't find it unobstrusive like you say, and regularly displays ads for the pay version (this doesn't happen in MS Security Essentials).
Last edited: Dec 14, 2011