If its simply a registry problem, you should be able to get around it the following way:
Rename your existing Train SImulator folder to something else (msts backup is a good option);
Uninstall msts, reboot and reinstall a clean copy from the CD's. This will replace the registry entries;
Delete the new installation (not Uninstall - just delete the folders)
Rename your old installation back.
This will have fixed the registry entries. Now you will have to make sure trainstore is pointing to the correct location, you do that from trainstores tools i think from memory. Then do an unstore all to synchronise your trainsimulator folder with your train store folders.
Reboot your PC and try a single activity using easy trainstore.
Dont ever use system restore as that is known to get things in a tangle
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