That's a great tool there. I ran across that one several years ago with a 98 machine and still use it on Vista. Keep the link there as a reference. Several have had problems removing AVG when the uninstaller fails unfortunately. Other programs like Trend Micro's PC-cillin will automatically remove other programs for you when it installs but apparently not Avast.
If you look up to the very top left and open "tools" you will also see "registry cleanup". Clicking on that will open a dropdown menu to the right for the automatic cleaner there. I simply use the "do them all" option for letting it locate and remove anything left in the registry by uninstallers.