I think your question might get more help if you posted it on the Avira forum.
I have used Avira for around 12 months and I have never had any problems whatsoever. The problems you have had make me think it was some incompatibility with something else you have running, especially when you say "i rebooted in safe mode everything was fine".
Also, 3 hours to do a scan is a massive amount of time, I run a full scan on mine and it takes approx 30 minutes and that's on a system that has 130GB of stuff. You either have a hell of a lot on your computer, or your hard drive is in desperate need of a defrag. Even an older machine I would have expected it wouldn't take that long has Avira is very light on system resources.
Here's the link for the Avira forum, you will need to register first before you post a question but this is free and only takes a few minutes.