Switch to the deinstallation and deinstall XTC by using the provided installer/uninstaller, or from the original windows procedure. Both works as XTC is using a valid installer. Now go to your downloaded file again and install it by double clicking on it. You'll have X3tc installed in a custom game folder, but the installer is assuming a certain path to your game folder. You need to make sure that you install it into the right folder. So search your HDD for the x3tc.exe and write down the name of that folder and the complete path leading there somewhere. While installing, the XTC installer will ask you to install the files into a folder, which will be different than the one you have just found containing the x3tc.exe. Now type in the complete path to that folder, or hit search and choose the right folder from the menu. If you start the game the next time, the starting screens will show the XTC logo It's probably just because the files were installed into a wrong path.