From the VMware website.

On HP Proliant systems, when attempting to upgrade ESX 4.0 to ESX 4.0 Update 1 (U1), you may experience these symptoms:
- Upgrade operation may fail or hang and can result in an incomplete installation
- Upon reboot, the host that was being upgraded may be left in an inconsistent state and may display a purple diagnostic screen with the following error:COS Panic: Int3 @ mp_register_ioapic
WARNING: Rebooting the host means the host has to be reinstalled because it is not recoverable after a reboot.
WARNING: If you have virtual machines running on local storage, they may not be retained if you reinstall ESX 4.0 as a result of this issue. Contact VMware Support for assistance before reinstalling.
Solution
To avoid this issue, prior to the update, disable HP Insight Management Agents running on the ESX 4.0.0 server.
HP Insight Management Agents can be disabled using these commands:
- service hpsmhd stop
- service hp-snmp-agents stop
- service hp-health stop
HP management agents can be enabled after the upgrade is completed. Refer to HP documentation on how to re-enable these management agents.
If you have already updated the ESX host, do not reboot the ESX host. Open a support request with VMware support. For more information, see How to Submit a Support Request.