In some environments, VirtualMachineViewer and VMConnect always prompt for credentials. In order to avoid the error below, follow the instructions in this blogpost:
[Edit] Some customers have also found this post to be useful in resolving this issue: http://mindre.net/post/Your-credentials-did-not-work---Hyper-V-in-a-domain.aspx
The actual error is:
Your credentials did not work
Your system administrator does not allow the use of default credentials to log on to the remote computer <computer name> because its identity is not fully verified. Please enter new credentials.
Entering new credentials works in all cases, but it is a hassle :)