I bought RTW Alexander online from www.ebgames.ca, and when i try to install it, it says "No prior installation of Rome - Total War was found. This installation program will now terminate."

Ive looked at many forums and the totalwar.com site for solutions, and they all suggest to patch my game but mine already was. i tried reinstalling just RTW with and without barbarian invasion patch only 1 or the other and both but i always get the same error. I even tried all this on my bros comp and oops, i lost his saved games. Can anyone help me?