I have now rewritten the information on the web page I talked about in my previous post. I hope it is easier to understand now.
Just to be sure I'll guide you through step by step:
-Download both files from the downloads page
to your computer
-Right-click on the file called ROPv22_final_b.exe and select 'Run as administrator' from the menu
-Choose the folder where you want to install to, and then install. (It should be the folder which has Rome - Total War v1.5 installed and there should be no other mods in that folder changing the original files.)
-Right-click on the file called ROP2_22b.exe and select 'Run as administrator' from the menu
-Choose the same folder as you have chosen before and install.
-Play ROP by clicking the 'Rise of Persia' shortcut on your dekstop