If you've opted for a paid utility to keep virus, trojans, and other threats at bay, you should first uninstall Avast Home so you can install the new antivirus tool.
It does what Norton Removal Tool does with Symantec applications. Avast Home has an uninstall utility that completely deletes the antivirus and other tools from the same creator (ALWIL Software) from your computer. The name of this utility is Avast Uninstall Utility.
The user just has to select from the drop down menu the name of the application that they want to uninstall. Avast! 4 Home/Professional/Server, Bart CD Manager, Avast! PDA Edition, Avast! Network Client, and Avast! 32 appear on the list.
If you don't remember the name of the Avast! product that you want to delete, Avast Uninstall Utility allows you to select the directory where the application was installed.