If it's in the market, a virus is not the issue. But to put your mind at ease, try "Lookout", it's free and scans all apps, in addition to providing backup. "Lookout" is more for folks who download off-market apps, as I doubt Google doesn't check apps before going live in their market.
Either way, I was frustrated with Qik...always froze, disconnected and you could barely hear the other person...not pleased. Also the contacts and calling people on QIK was always buggy. I would buzz a friend and he never received any indication of incoming calls, even when hand typing his QIK username.
Tango has worked perfectly so far and the video / audio is significantly clearer. I tried calls with a friend (who also has an Evo) on WiFi, 4G and 3G. Obviously the WiFi is going to be the clearest and most stable option. No hiccups, perfectly clear and does look a lot like FaceTime.
4G was also fine, you could see a slight difference in video, but nothing distracting.
3G was more of a challenge. Calls mostly stayed connected, but there would be pauses in the audio and video and you see a little more pixelation during movement. 3G would work if you needed it, but I would always default to 4G or WiFi if available / best experience.
One thing that was weird, I am assuming specifically on the EVO, is that if both of you put the phone down in landscape / on the kickstand, one of you is upside down in the video. Hopefully they read the forums and fix this.