and i am guessing that when you try to uninstall it cant because the uninstaller file that was in the CIV4 Directory is missing.
Go to http://www.tune-up.com and download TuneUp Utilites 2008. Use the registry cleaner utility that is included. it will find the registry entries that were left by CIV4 and will allow you to remove them (effectivly uninstalling the program) Once done you will be able to reinstall Civ4.
This process works with other programs as well.