There's an old post (2010) with the same error message. The issue there was a conflict with a third-party firewall. Did you update your antimalware/antivirus/firewall software between the time you were able to run XP Mode and the time the error occurred? See https://answers.microsoft.com/en-us/windows/forum/all/virtual-machine-error-server-execution-failed/41177b07-1c98-4772-ae51-4c9e331cbc24
For what it's worth, the downloads for XP Mode and Windows Virtual PC, although no longer available from Microsoft, can be downloaded from the links in this thread, which also would be a good place to ask about why reinstalling Virtual PC isn't working: https://www.sevenforums.com/tutorials/8247-windows-xp-mode-install-setup-37.html