Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
Since you also have a problem with uninstall, it is possible that your installation is somehow corrupt.
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
If that doesn't work try, uninstall (see below), reboot, install, reboot.
Because of the uninstall problem you experienced, download the avast! Uninstall Utility,
find it here and save it to your HDD. Now uninstall if that fails again use the uninstall utility, reboot install the latest version, reboot.