Hi Peter,
have added the registry hack as per 3ware and that "fixes" it, the guests are no longer crashing on login. The actual error is happening on the host though. It writes the errors as stated by the thread opener into the syslog on login into one of the guests, either through Hyper-V or RDP. Login crashes the guest on initial ESET scan and writes the errors on the host. I assume this happens when ESET is trying to access the VHD in some manner it doesn't like. As the registry change works around it this must be related to unaligned buffers. The guest then bluescreens with "critical process died". As the host isn't crashing I can't send you meaningful logs. If you have any ideas on how to debug this I am game to help.
Thanks