a) I would trace the installation starting with the McAfee Agent log and seeking the section where it starts the HIPS installation cient task, if it says successfull, then I would seek the actual MSI log file that were produced locally on client as a result of the HIPS installation. this log file should contain any error that might have occurred.
b) Usually the windows log file shows what dependenciy service did not start and why, if not I would say the dependency service file is missing.
c) OAS is disabled, first I would seek any McLogEvt entry in Windows Applicaton log. Otherwise I would Repair the product from Add/REmove Programs.