All you have to do is make a copy of your RTW Vanilla (original/unmodded) installation. No need for partitioning drives or multiple os. Unless of course you want to partition your drive to just play games on that partitioned drive but that's another story.

To make a copy of your Vanilla RTW installation go into
C:\Program Files\Activision or Creative Assembly\Rome Total War
Here make a copy of your entire RTW folder and you can rename it to whatever you'd like. For example if you are going to install Europa Barbarorum on this copy you can rename it so..or if your installing RTR Platinum you can do so as well. That way when your actually installing the mod you can point the mod's installer to the directory you want to use. You can have multiple copies like this on your HD presuming of course you have the space. There are mods that use the mod switch but for the sake of simplicity i won't get into that.