George,
I think it all is a bit of a scam myself all these anti virus products but I have had a few virus's & trojans in my time (All when I had paid for AV's installed). I have often wondered if Norton, Mcafee etc make these virus's up so we buy there products, who knows :

.
As someone said earlier, I think it was Santa, Microsoft security essentials is free and quite good by all accounts. Now as it is free Microsoft in theory have nothing to gain from making it. So again why make something for nothing? is it because there really is very little out there to worry about and the cost is minimal to make a product like this? and it shines a bit of a light on the Microsoft empire?
Anyways, as its free why not use it? nothing to lose.
I too reinstall everything every 6 months or so too.
PS, pleased to here you got rid of whatever was on your puter Axe ;D
cheers
Will