The order is the thing to watch - download avast and save it to your hard disk (where you can find it again) before you uninstall McAfee. That way you aren’t on-line without protection - uninstall McAfee as advised above and reboot - now install avast.

Download location for avast! Free AV: - http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe