Since you have implemented John Howards stuff we will assume that either your Hyper-V server or your Vista PC is in a workgroup.
OneCare on the Vista PC should not cause the issue unless the firewall is blocking the return traffic for some reason.
Check the OneCare firewall settings and make sure that traffic is not being blocked.
You also have to check your user accounts and passwords (login as the same local user and password on each box).
You are not using AD, but htere is still some level of passthrough authentication that is happening in the background.