Lookout and AirPush Detector have always been my go to for my Android devices, so I use them on my KF as well. For browsing, I use Dolphin browser and TrustGo Security and an add block extension. I haven't had any problems with viruses or malicious apps so far. Actually, I had an app once that started putting adds in my notification bar and desktop. That's when/why I started using AirPush Detector. It identified the app immediately and uninstalled it. I had been trying to figure out which app it was and how it got past my security precautions for a while before I came across AirPush Detector, so I highly recommend it in junction with Lookout and a decent web browser.