Another member on this forums ran into this issue:
https://forums.totalwar.org/vb/showthread.php?t=55747

Since I'm new I cannot post there yet, so I thought I post here.

If you get this error when installing the 1.3 patch do the following:

1: go to the C:\Program Files\Common Files folder
2: Locate the InstallShield folder
3: move this folder to the desktop, or delete it
4: try installing the patch again.

This solved the problem for me.

Hope this helps out.

Now I just fininshed the tut so I'm off to start a proper campaign and risk my relationship yet again no doubt. :)