Download all files into a single directory. Open VS Code. If not already installed install the Live Server extension. Browse to main.html.
Unzip into a root dir in your C:\ drive. Yes it a windows app. You do not need to run any install, unzipping is sufficient. Right-click the Xselerator.exe, and click the Compatibility tab. Select Windows 7.
Also select Run as Administrator.
I also always get the message box "CoCreate failure on IAltovaApplication" on start. Just click ok, and ignore it, Xselerator.exe works fine. Strange - I even get this on machines that do not have Altova XmlSpy installed.
It has been a while since I installed Xselerator on a fresh machine. I do seem to recollect getting another message but can't for the life of me remeber what it was.
Maybe to do with missing Dom element? If so try installing msxml4 (included, msxml4.msi) If you don't trust this one you should be able to still obtain it from the Microsoft site.. maybe.
If there's any interest I'll update with additional recommendations in the comments.