Right, that looks obvious now. I tried it but get no more information:
StartLog:
09/06/13 17:03:33 ** condor_startd.exe (CONDOR_STARTD) STARTING UP <===restart everything
09/06/13 17:03:33 ** C:\Condor\bin\condor_startd.exe
...
09/06/13 17:25:57 slot2: State change: SUSPEND is TRUE
09/06/13 17:25:57 slot2: Changing activity: Busy -> Suspended
09/06/13 17:26:02 slot1: State change: SUSPEND is TRUE
09/06/13 17:26:02 slot1: Changing activity: Busy -> Suspended
09/06/13 17:26:02 slot2: State change: CONTINUE is TRUE
09/06/13 17:26:02 slot2: Changing activity: Suspended -> Busy
09/06/13 17:26:07 slot4: State change: SUSPEND is TRUE
09/06/13 17:26:07 slot4: Changing activity: Busy -> Suspended
09/06/13 17:26:07 slot1: State change: CONTINUE is TRUE
09/06/13 17:26:07 slot1: Changing activity: Suspended -> Busy
09/06/13 17:26:12 slot3: State change: SUSPEND is TRUE
09/06/13 17:26:12 slot3: Changing activity: Busy -> Suspended
09/06/13 17:26:12 slot4: State change: CONTINUE is TRUE
09/06/13 17:26:12 slot4: Changing activity: Suspended -> Busy
09/06/13 17:26:17 slot2: State change: SUSPEND is TRUE
09/06/13 17:26:17 slot2: Changing activity: Busy -> Suspended
etc.
and the condor_config:
MachineBusy = ($(CPUBusy) || $(KeyboardBusy) || $(ConsoleBusy))
MachineNotBusy = ($(CPUIdle) && $(KeyboardNotBusy) && $(ConsoleNotBusy))