Hyper-V Virtual Servers will not start - 'Servername' Failed to start worker process: class not registered (0x0040154)

This morning, virtual servers on one of my Hyper-V host servers were all in the 'SAVED' status.  When I try to start them I get the following message (full details):

An error occurred while attempting to change the state of virtual machine 'ServerName'.
'ServerName' failed to restore virtual machine state.
'ServerName' failed to restore virtual machine state.  (Virtual Machine ID .....)
'ServerName' failed to start worker process: Class not registered (0x80040154).  (Virtual Machine ID .....)

I have attempted system reboots, backed out the 4 patches installed last night (all released 11/11/2008), checked permissions on the files, and tried to recreate the virtual servers using the existing .vhd file.  I also tried creating a new virtual machine from scratch and it gives the same error.
Has anyone seen this error before and know how to correct it?

Server is an HP DL360 G5, latest firmware and Bios running Windows Server 2008 Enterprise Full install with Hyper-V role only.  No A/V or other apps installed!

To make things worse, I rebooted one of my Server Core systems running Hyper-V (all it's VM's were running) and after the reboot it is having the same problems as the system above!  It also patched last night but all VM's were running this morning.

Thanks!

This error is also listed in the Hyper-v-VMMS log.
SCWestServerAsked:
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SCWestServerAuthor Commented:
FYI - More info.  This server is running the full version of 2008.  I also have a Core Server running Hyper-V whose VM's were running.  I rebooted it and now the VM's on that server also will not start and have the same error message.
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SCWestServerAuthor Commented:
All is well now, here is what I did:

I had to rebuild the OS partition of the Full Install Hyper-V host because Hyper-V did not uninstall properly and the system hung.  I was then able to recreate all the VM's using the existing VHD's.

On the server core system I uninstalled Hyper-V successfully and then reinstalled it.  I again had to recreate the VM's using the existing VHD's and they all started fine.
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