Hello, see below for the official work around sent to me by Nimble Giant Support -
Please, download the following file:
http://download01.regnumonlinegame.c...ROLauncher.exe
and replace the current ROLauncher.exe file in the installation folder of the game.
After this, do not run the launcher yet. Now open ROLauncher.ini with a text editor and make sure you have these two lines in there:
[build]
alternate=2
This will avoid updating to the current version that is not working in Windows XP.