Since the installation process is causing quite a lot of issues, and the "2.0 + 2.5 patch" install is rather complicated, I've created a new installer which combines everything in one file.
There is nothing new in it, that is none of the hotfixe that might have been released after the 2.5 patch.
This is not RS 2.6 or a new patch !
To install it, simply download the exe (about 1.3 GB), and double click to launch it.
The installation process should be straightforward.
Download link :
http://www.moddb.com/mods/roma-surre...a-surrectum-25 (use this one if the bandwidth is spent on the first one)
Here are the complete installation steps for RS 2.5 using this installer.
1 Have a clean RTW install (that is, no mods installed in it or any modified files) patched to 1.5, plus any add-ons as applicable patched to 1.6 for BI and 1.9 for ALX.
If you are unsure your install is clean, the best thing to do is to try to start a campaign.
2 Copy that folder, and paste it to the location of your choice.
Do NOT put it in the Program files ! (For Steam users who have no choice, be aware that you may encounter some issues with the launcher.)
The best thing to do is to create a new folder at the root of your hard drive (C:\ usually) called games, and copy/paste the RTW folder there.
The resulting path would be C:\Games\RTW.
3 Check that inside this RTW folder, you have the exe(s), as well as several folders including data.
You might want to rename the RTW folder to something like Roma Surrectum II.
The new path is going to be C:\Games\Roma Surrectum II.
4 Double click on the installer called Roma Surrectum 2.5.exe (you might not see the .exe part if you have the file extension view disabled in windows).
5 Once you the installer asks you where to install RS 2.5, you'll have to click on the "Browse" button, and select the Roma Surrectum II folder we have just created. Click ok.
Under "Destination Folder", you should now see the correct path, that is in our case : C:\Games\Roma Surrectum II.
If you are in the least bit unsure about the installation path and this step, it is highly advised that you follow exactly the steps as described above.
6 Now click on the next button, and change the default name of the Start Menu item if you so desire.
It is advised to leave it as it is.
7 The mod should install itself, as well as create a shortcut on your desktop.
You should now be able to play RS 2.5.
Warning : It has been reported that the launcher does not have admin right by default.
If you encounter troubles with the launcher, go to C:\Games\Roma Surrectum II\launcher.
Right click on the Roma Surrectum II.exe file, select properties.
Go to the compatibility tab (the second one from the left in WIN 7), and check the "Run as an administrator" checkbox.
Apply the change, and it now should be ok.
Here is the original post for RS 2.0 where you can find more information about the mod.