Quote Originally Posted by Dol Guldur
That's simple. You just copy everything in the folder C:\Program Files\Activision\Rome - Total War (or your PC's version of that path) into one of your own naming such as C:\Program Files\Activision\Rome - Total War Copy

It takes some time to copy over but once you have it done then you can keep the original as vanilla (and a backup) and use the copy folder as a source for mods (or for one mod if you wish to install one that overwrites the game files such as Roma Surrectum).
I might need a step by step tutorial on that.