in fact I wasn't the author of this mess, it was a friend of mine (or his children) who installed avast free without first unistalling Norton Internet security, I mean he just deleted some files and that's it. Because of this I downloaded the corresponding norton tool to remove properly this suit of security, then I ran a maintenance program to make sure there was nothing on the way for avast to take care of the system. Finally I tried to repair the Avast's installation but it didn't erase those weird entries in the scan tab, apparently the real-time protection works fine because I created a fake virus and it was detected immediately.
thanks to all of you for the contribution...