Angus I agree that is a good next step. Here are a few of the events I see in the event log when I stop and start the ArcGIS Server Service.
Error - Event ID 8198 - License Activation (slui.exe) failed with the following error code:
Information - Event ID 4, 5,6,7 (All with the following text)
The description for Event ID 6 from source ArcGIS Server cannot be found. Either the component that raises this event is not installed on your local computer or the installation is corrupted. You can install or repair the component on the local computer.
If the event originated on another computer, the display information had to be saved with the event.
The following information was included with the event:
ArcGIS Server
The specified resource type cannot be found in the image file
I'm starting to decipher these different event ID's.