how can i uninstall when i cant do it by add/remove programs i windows?
If I can ask why you need to uninstall, perhaps we can help ?
It would probably be best to first 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.
Now run the avast! Uninstall Utility, reboot, install the latest version, reboot.