If you have a problem connecting to a Hyper-V VM with error: Cannot connect to the virtual machine because the authentication certificate is expired or invalid or when you try to start a VM and an error that YourVMNAme failed to initialize with an Unspecified Error “0x80004005”. This is because the Hyper-V Management Service certificate has expired.
This is very fixable by just applying an update: KB967902 for Windows Server 2008 x64 Edition. So if you have your WSUS administration or its a standalone server, better apply this hotfix to prevent what happened to me or to fix this issue.
More info is available where I found the fix here: http://support.microsoft.com/kb/967902 or download the hotfix directly here: http://www.microsoft.com/downloads/details.aspx?FamilyID=289c831c-9142-4c00-bacd-6c1924ff0ecc