I know this is an OLD thread here, but I'm gonna throw my two cents in. First off, you should never use more than ONE antivirus program on your computer at any given time. Uninstall everything else. Same thing for Anti-spyware programs. The reason is that they tend to conflict with one another and cause issues on your PC. Now, as for the quality of ADVAST and Avira and AVG, the only one I have experience with is AVG FREE and the purchased version as well, both were complete crap. I found spyware on my PC, as well as Trojans, when I switched to Kaspersky Internet Security. KIS is a full suite with firewall, anti-spyware, and anti-virus, all rolled into one nice program. And IT WORKS WONDERS. Bought Norton Security Suite right before that, and I'll be damned if Norton didn't leave some spyware and trojans on my PC because they were "Pre-existing" programs. It actually recognized them as legitimate software because they were already on my comp. Now, I know this sounds like an advertisement for Kaspersky, and in some ways... I guess it is. But I honestly cannot say enough good things about Kaspersky, because it has not let me down one single time, and pings the hell out of ANYTHING that tries to install itself on my PC.

That's just my recommendation, take from it what you will.