Firstly... almost anything people install is third party, mostly coded by some random guy on a forum, which means a very high chance of false negatives, so you can never really tell what's a virus and what isn't.
Secondly, as said before, there are basically no actual viruses.
Third, I could post a cab right now saying its the best antivirus available, name it Norton, and you'd install it, having it run on every boot and send me your contact list.
Fourth, you can completely reset your device to a perfect state in less than 2 minutes and 5 key presses.
Fifth, the phone's battery is bad enough to not run a constant scanner in the background.
You can do whatever you want, but I would never pay for something that is worthless in my opinion. The PPC software subforum has a few threads with many links, I'd look there
