Well we are on version 4.8.1290 so I would suggest a clean reinstall (see below) of the latest version. So it really is best to have the latest version of avast as there is little value in trying to fault find with an out of date version.
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
1. Now uninstall (using add remove programs, if you can't do that start from the next step), reboot.
2. run the avast! Uninstall Utility, reboot. If step 1 failed it may be necessary to run this from safe mode, once complete reboot into normal mode.
3. install the latest version, reboot.
