automatic shutdown after scan complete

I have a problem where my system will sometimes shutdown if I leave it running overnight. I have run into this problem a couple of times and narrowed it down to:

The process winlogon.exe has initiated the restart of SUPERSPORT for the following reason: No title for this reason could be found
Minor Reason: 0xff
Shutdown Type: shutdown
Comment: avast! Antivirus - scan complete

I checked the event logs and it shows this error 3 times in the last 5 weeks, which sounds about right. The system is a desktop and set to never sleep or hibernate, so it looks like it is avast! completing that is causing the shutdown. Has anyone ever seen anything like this ?
Thanks

Hallo,
seems like you’re in a wrong sub-forum - this has probably nothing to do with the Mac version :).

Regards,
pc