This issue may occur if the VirtualCenter Server service starts before one of the services it depends on is started. The VirtualCenter Server service is dependent on the following services: SQL Services ADAM Services when using linked mode in vCenter Server 4.x To resolve this issue, add a dependency to the VirtualCenter service so that it starts only after the necessary services have started.

via VMware KB: VMware VirtualCenter Server service does not start automatically after reboot.

Yikes!  You need to edit the registry to get this to work.