Win 7 32-bit, Ult
It's not from the tray but the Start Center that the scan begins automatically. I understand that a "system scan" can mean the entire system to some people, so then there should be an alternative for a more limited scan. Most AV's I've used give you a choice of drives to be included in a system scan.
The alternative would seem to be the file/folders scan, but specifying a drive there just locks the program. Even before it begins a malware scan, it does some sort of scan of the whole drive, possibly to determine the status and size. So, it actually ends up scanning the drive twice if it ever completes, which it has rarelyfor me. Sometimes, it seems to finish, but nothing appears in the window and I'm back where I started. Also, the file/folder scan has no option for anything except "Remove Malware", so that doesn't give you the chance to choose what it removes.