My (rough) understanding of antimalware is that scans are automatically run using some special account that has (almost?) universal access. The System account comes to mind from past readings about this.
From what I observe, I am concerned that this is not what happens for Spybot S&D. When I start a Spybot S&D scan from a nonadministrator account, I do not see a special account doing the scan in the task manager. I sort the processes by CPU usage and SpybotSD.exe is the majority user by far. However, it is run under the non-administrator account. Does this mean that it will not scan as thoroughly as running it from an administrator account?