P/E 2850 Hangs on shutdown / reboot

Hi,

    I have three poweredge 2850 server with windows 2003 installed ( 2 are citrix servers and one is an ISA server). If we choose to shutdown or reboot these three machines as windows closes we get a grey screen with an active mouse pointer and nothing else, they have been left like this overnight and are still at the same stage in the morning, the only option is to cycle the power after which they boot and run fine until the next shutdown or reboot, Any ideas ?