One step more: Use avastclear.exe to completely remove any remnants left over.
https://www.avast.com/uninstall-utilityBe sure to move the downloaded file to your administrator desktop before running this tool. It will ask if you want to run Safe Mode when you double-click. Click 'Yes' and your Vista system should automatically reboot to Safe Mode. Be sure to leave the default cleaner settings alone unless you installed avast! in a non-default folder. In that case, you would point aswclear.exe to that avast! non-default folder. If you installed to the default folder as you should you do not need to do anything as it is already set to remove the avast! default folder location. Select what version of avast! you wish to uninstall. Reboot when done.
If you've ever run or installed another a/v solution, or Vista came with a trial version you uninstalled later, see:
https://singularlabs.com/uninstallers/security-software/Download any vendor tools as appropriate. Reboot after running each tool. Set a restore point before running any tools to recover if necessary, but should not be a requirement to do so.
If any old a/v's were run and then uninstalled later, then these left-over files could be the source of your issues with avast!.