THIS IS DANGEROUS.
Press WindowsKey-R to get the "run" prompt.
Type "regedit".
Now, very carefully, move down the registry:
HKEY_LOCAL_MACHINE::SOFTWARE::Firaxis Games
Select "Sid Meier's Civilization 4 - Beyond the Sword"
Open the EDIT menu. Select Delete. Say "yes, I am sure".
Now close regedit.
Next, press alt-R again.
Type in:
"c:\Program Files\Firaxis Games\Sid Meier's Civilization 4\"
press enter.
Look for the directory "Beyond the Sword". Press delete.
At this point, pretty much all traces of BTS will be gone from your system. This includes every mod you downloaded. Reinstall the game from your CD, then download the 3.13 patch.
...
If you want to be more certain, you can also delete the civilization 4 and civilization 4 warlords stuff -- both the folder, and the entry in the regedit program. If so, reinstall in the following order:
Civ 4
Civ 4 Warlords
Civ 4 Beyond the Sword
The 3.13 Beyond the Sword patch.
Good luck.
Note: REGEDIT is dangerous. You can easily destroy your entire system with the slip of a key. Do not play around with regedit unless you have to.