I know this is an old thread but I found no answers elsewhere when I was looking for a resolution to my resolution issues so,
I had the same problem in both Vista and Win 7 of only having 800 x 600 in the Strategy map and 640 x 480 in the Battle map.
A fix I found that worked with a GTX 280, GTX 295 and an HD 4870 X2 is:
Got to
All Programs open the
Accessories folder click on
Run type
MSCONFIG in the pop up window and click
OK. If Windows asks for permission click
Continue. In the new pop up window select the
Boot tab and click on the
Advanced options... button and in the new little pop up window click the tiny box by
Maximum memory: Whatever the number in the box here change it to
2560 then click
OK. Click
Apply on the remaining window then a small pop up asking you to restart PC should remain, click
Yes. You should now be able to play
both Shogun Total War and Medieval Total War with all resolutions available.
To revert your system to normal you have to uncheck the box in the
Advanced options... ,
Maximum Memory: click
OK then select the
General tab and click the little round button beside
Normal startup Load all device drivers and services then click
Apply and then click
Yes when asked to restart the system.
What you are doing here is only allowing the system to let the games software see an amount of RAM it is programmed to understand, or so I believe, thus preventing it from defaulting to minimum specs. If the number 2560 does not work for you in the Maximum memory box try 2048. I just found on my rigs 2560 was the highest amount I could go and still get all the resolutions.
I should also mention that for the HD 4870 X2 I had to disable AI in the ATI Catalyst Control Center and in all cases I applied the "button fix" for Medieval Total War.
I believe/am guessing that those who never had issues running Shogun Total War or Medieval Total War in Vista/Win 7 had 2.5GB or less RAM installed??
(most excellent smilies here!!!)
Good Luck!