Possible reasons are:
There have been cases where the absence of an active KVM connection/keyboard caused Windows to stall. Make sure you have an active connection to the machine and press the shift key on the keyboard.
A broken MBR can cause this. To fix this, run "format c: /fixmbr" in a Windows command shell.
There is a known bug with Microsoft Windows Server 2003 (see https://support.microsoft.com/kb/834100/) that prevents the installer from rebooting a remote host if nobody is logged in on the console. This can cause problems when upgrading hosts from the cluster manager. Microsoft provides an unsupported tool that allows the machine to be configured to automatically log in a user on the console when the machine reboots.