Unable to install a avast antivirus

This usually happens when there is an existing anti virus present. Make sure you remove your old anti virus completely. The best way to do this is by going into "Safe Mode" turn computer off. Turn it back on and continually press F8 you will eventually get a black screen with a bunch of options. Go to "Safe Mode" and press enter. Once the computer starts, remove old anti virus using "add Remove programs" in the control panel. Now you should be able to install the new anti virus. Hope this helps.