What version of avast are you using and what is the build number, e.g. 6.0.1203 ?
What other security software do you have installed ?
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
@ maurizio3329
The current beta is quite stable, certainly on my system. So if you want to try it, see this topic, http://forum.avast.com/index.php?topic=83583.0 for information and download links. Or you can wait for the official program update release, which hopefully shouldn’t be too long.