the chances are you cannot uninstall because it is set to switch on when you boot up. You do not say what op sys you are using. Assuming XP go to start - run and type in msconfig click on return and then click on the top right tab - startup. Find the name avast and click on the left hand box with tick in it to remove the tick. Reboot the pc and avast should not load you can then delete it by using add/remove progs in control panel.