So for the last month or so I've been running Microsoft Security Essentials and I must say I'm impressed, I was very sceptical at first about a Microsoft anti-virus/malware program, however they got it right.
It's lightweight, unlike a lot of AV programs, even the old much beloved AVG feels like a 10 ton brick compared to MS SE, which for me is very important, I freaking hate programs like Nortons which drain more out of my system then any virus could.
It's easy to use, which isn't much of a factor for me, but at least it means it was easy to setup.
And best of all it just plain works, and doesn't bug me.
So I give it a 4/5, if you need a new AV product grab this, it's free (if you've got a legit version of Windows), and very easy to install and use.