The first thing I would suggest is to update your version of avast as it is well out of date. The latest version is 4.8.1201, so there is little value in trying to track down any issue in an old version.
If your virus signatures are equally out of date then viruses have a strong possibility to get into your system
The same is true of your OS (which is ?) and other security based software, it must be kept up to date. A visit to
http://secunia.com/software_inspector/ would be in order, as this chesks your system for a number of applications to see they are up to date.