On my older installs, I use ESET NOD32 AV, which gives great protection, while not being a RAM hog. But their IS was too much for me to pay. Therefore, yesterday I installed Avast Internet Security, 3PC's for $29.99. In order to get it this cheap, I had to install the free version of Avast (I did this on XP Mode). After a couple of weeks, the 3PC for $29.99 showed up, and I bought it.
So far, I'm satisfied with Avast IS, I had to do a little tweaking for XP Media Center to get the lead out of it's butt, but it's OK now. I have a copy of Avast IS installed in Win 7 Pro x64, Vista Ultimate x64, and as already mentioned, XP Media Center. The installs on 7 & Vista went perfectly, I feel that I had issues on XP Media Center because the 32 bit OS cannot use all of my 4GB RAM.
I've used the free version of Avast from 2006-2009, prior to MSE's release, it served me well. Plus I've ran it in various VM's since then. For the money, 3PC protection for $29.99 is not bad at all. That was the same price that I paid for the 3PC ESET NOD32 AV. I wanted ESET Smart Security, but knew that I had no chance to get it for that low of a price. With 6 Windows OS's to maintain, I have to do the best that I can.
And I really no longer care to run free AV's, because most has paid counterparts that always leaves the best components out.