The smallest "install file" would be a batch file that created a Horizons folder then generated a reg file that inserted a value that pointed at the new folder. The next time you went to log in via the website, all game files would be seen as missing, and downloaded.
Mostly off the top of my head, I think it would go something like:
Code:
@Echo Creating Horizons folder.
@MD "C:\Program Files\Virtrium"
@MD "C:\Program Files\Virtrium\Horizons"
@Echo Creating registry file.
@Echo Windows Registry Editor Version 5.00 >> temp.reg
@Echo [HKEY_LOCAL_MACHINE\SOFTWARE\ArtifactEntertainment\Horizons] >> temp.reg
@Echo @="C:\\Program Files\\Virtrium\\Horizons" >> temp.reg
@Echo Importing registry entries.
@regedit /s temp.reg
@del temp.reg
@Echo Launching IE. Use the web based patcher to complete the installation.
@start iexplore http://play.istaria.com/
@pause
Eventually I suppose the ArtifactEntertainment registry path will change to Virtrium, and the Horizons folder will be Istaria (at least for new installs)