I am playing M2TW again after many years. A long time ago, I used King Kong's SS (sans Kingdoms) on a disc installation. Had no troubles, life was good. Fast forward to the present day, I got M2TW and Kingdoms on Steam so I don't have to mess with CD dongles. The newest version of SS for Kingdoms is nothing like what I used to love so I looked for another mod. Found this one. Downloaded the installer. Computer told me I needed Kingdoms installed. Oh... it's gonna be like that, huh? Great.
Spent a couple days of absolutely intense frustration searching the internet for solutions that do not work, solutions that are missing a vital step that you can't proceed without, and solutions that are just... useless. So I just figured it out myself. You need to edit your registry so these installers know where to find your installation. I use Windows 8.1 64 bit. Most likely these instructions will be similar from Vista to 8.1
WARNING:
If you do this and mess up your computer (and you can seriously brick it if you don't pay attention), that's your own fault. Read carefully.
Hit Start
01. Put 'regedit' in the run box.
02. Go to HKEY_LOCAL MACHINE\SOFTWARE\Wow6432Node\SEGA\Medieval II Total War
03. Right click in empty space somewhere on the right pane.
04. Select New > String Value
05. Type 'AppPath' without the quotes and hit enter
06. Right click 'AppPath' and select Modify...
07. In the bottom box put your M2TW install directory. Example - C:\Program Files (x86)\Steam\SteamApps\common\Medieval II Total War
08. Right click in empty space somewhere on the right pane.
09. Select New > String Value
10. Type 'language' without the quotes and hit enter
11. Right click 'language' and select Modify...
12. In the bottom box put your language. Mine says 'english' without the quotes.
13. Right click in empty space somewhere on the right pane.
14. Select New > String Value
15. Type 'version' without the quotes and hit enter
16. Right click 'version' and select Modify...
17. In the bottom box put '1.03.00' without the quotes.
18. Close the registry.
19. Try the KGCM 3.0 installer again.
I'd also suggest using a 3rd party launcher if you get never-ending music with the stock version. I use this one.