1. Are you sure this is an avast message about ashMaiSv.exe and not either your firewall or other security software ?
What other security based software do you have that might block new startup entries, e.g. Spybot S&D (TeaTimer), AdAware (AdWatch), SpySweeper, PrevX, WinPatrol, ProcessGuard, etc. ?
2. I trust you did reboot and the prompt keps coming ?
This could be solved, in some systems, by deleting the file C:\Program Files\Alwil Software\Avast4\Setup\reboot.txt