A few gotchas I've found doing a simple mod to GlobalDefinesAlt.xml in Vista:
1) If you use an XML editor like XML Copy Editor, make sure the file is being saved in Windows (CRLF) as opposed to Unix (CR) line termination format. Civ IV will throw a syntax error on the XML file if it is not in windows format.
2) If you attempt to save the file directly in \Program Files\Firaxis Games\Sid Meier's Civilization 4\Assets\XML, it will instead save a shadow copy to your \Users\yourname\AppData\Virtual Store\Program Files\... etc. This file will not be readable by other users on the computer. In fact, it appears that if you originally installed Civ 4 with one ID and are running it under another ID it will not read the file in any case.
I did find that putting the file in \Users\myName\Documents\My Games\Sid Meier's Civilization 4\CustomAssets\xml worked fine once I got past the syntax error issue. Note that in addition to my own changes, I also set the value of the override define to 1. I'm not sure if this is required, but I don't see any reason to change it.
<Define>
<DefineName>MODS_SHOULD_OVERRIDE_GLOBAL_DEFINES_HERE</DefineName>
<iDefineIntVal>1</iDefineIntVal>
</Define>
In order to ensure you save in Windows format, you can use Notepad, but when you save, if you do "Save As" be sure to change the file type in the save dialog from Text file to All Files, or it will append a .txt extension.
I've found my best option is to use a one of several free text editors that do XML syntax highlighting and allow you to set the line termination format if that becomes necessary. Some examples are Programmers' Notepad (
http://www.pnotepad.org/download/ ) and Notepad++ (Sourceforge:
http://notepad-plus.sourceforge.net/uk/download.php ). These are both Open Source Freeware. As is XML Copy Editor...