EUREKA!
The game took a little longer to load, but I got the menu.
The thing is, apparently Win XP gets into trouble trying to open the .fpk files. Civ4 used to have them, but it seems the expansion packs and patches eliminated them. Wildmana, however, was still built within these files (if I had to guess, I'd say because of the huge amount of artwork it has in surplus to vanilla Civ, but Sephi and tesb and the guys have been doing such a great job that I blindly trust their reasons).
Anyway, I digress.... here's the solution:
http://apolyton.net/showthread.php/140859-New-fix-for-the-renderer-error?s=
Ignore whatever is says about requirements and all. This is the fix for games built on .fpk files running under Win XP, period. So:
1. Download the application on the link above (it's no virus, don't worry)
2. Get it out of the zip file and run it. Go to the "PAK" menu and select "Unpack"
3. You'll be prompted for a source file to unpack. Go into the "...\Wild Mana\Assets" folder and select all .fpk files you find. ("Three is the number thou shall count. Four thou will not count, neither will thou count five")
4. You'll then be prompted for a destiny directory to unpack to. Select the very same where you found those ("Five! - Three, sire. - Three!") files.
5. Wait. Just a couple minutes. You'll get no signal that it worked, but don't worry.
6. Move those three .fpk files out of there. It may be safe to store them elsewhere on your hard drive.
7. Enjoy the best Civ4 mod ever, on one of the worst OS ever.
(on a personal note, I'm actually not sure if the unpacking is necessary, or the files are already unpacked by the installation and all you have to do is move the .fpk out of there so as not to confuse Windows... too late for me to try, though)