Well done.
You should now have a new installation of MSTS that is not in any way affected by the Vista security protocols.
Next thing to do is to go into Train Store, then go to
TOOLS, OPTIONS, MSTS FOLDER..., select the folder where your new MSTS installation now resides (i.e C:\ Microsoft Games\Train Simulator).
Once you've done this, I would run MSTS in the normal way (through Train Store if that's how you do it) to test that it works as expected and that you still have access to all your old routes and downloaded stock.
Then, try installing your Dorset Coast CD - hopefully it should now install and run without a problem. Let me know if you encounter any difficulties.
There's also a trick you can use with MSTS Bin to maximise the available system memory that MSTS can use - Dorset Coast in particular can benefit from using this, as it uses a lot of system resources due to the complexity of the route. If you've not heard about this, let me know and I can talk you through it, it's a very quicjk and easy procedure.
Once you are satisfied that everything works OK, you can delete the RAIN SIMULATOR folder.
Best of luck!
Ash