I was doing an install of Windows 7 Pro as a Xen hvm and ran into this problem. It took a while to figure out, and I finally found this is not an Xen issue but a well known issue with Windows for Windows 7 and 10.

The installation goes fine, then the machine reboots and you get the message:

The Computer Restarted unexpectedly or encountered an unexpected error. Windows installation cannot proceed. To install Windows, click OK to restart the computer and then restart the installation.

Pressing the return key reboots the system, then it ends up back at the same place. To repair do not press the “Ok” button. Instead:

1. Press Shift+F10, which opens a command prompt
2. Open the registry editor by typing regedit
3. Find the key HKLocal machine/SYSTEM/SETUP/STATUS/ChildCompletion
4. Select setup.exe on the right pane and edit it, changing the value from 1 to 3
5. Close regedit and the command prompt
6. restart the system (with the “Ok” button)

See [https://answers.microsoft.com/en-us/windows/forum/all/error-message-the-computer-restarted-unexpectedly/b770f14d-e345-e011-90b6-1cc1de79d2e2] for full details (mainly people saying thank you to the solution).