Downloading mod packs can be dangerous, as they generally involve overwriting at a minimum your civ3mod.bic file. You need to make "backup" copies of all the standard files you replace before unloading the mod.
That said, almost all of them come with detailed instructions, and if you need more help I'm sure the author would be quite willing.
At a minimum, I'd copy your civ3mod.bic file. My copy is called civ3mod_117_orig.bic, and I've write-protected (marked it read-only using the properties page for the file) it also.
Cheers,
Shawn
