Doom 3 RoE Install help

Ok i have just baught Doom 3 RoE expansion pac and i try to install it and it works fine up untill i have to update my version of Doom. the setup minimizes and launches the patch, but while im extracting it it says "Your installation of Doom has already been updated to version 1.2. This patch cannot be restalled."

But i havent patched it to anything else apart from the basic installed version becuse i installed it again when i got the expansion. Then after this error the patch closes and about 5 seconds later the RoE installer closes.

i have tryed everything manually installing it buy extracting the files from the discs straight to my hard drive. ive tryed updating it from within the game. and i have tryed installin only part of the game but nothin seems to work. can anyone help me because i am getting very frustrated.