Originally Posted by CIARAN
If you want to be on the sae side (and have the HD space to spare) you should copy your whole installation folder before installing any mods (which either come with self-installers or are files that have to be copied to a certain directory, which one is usually stated in the mod readme), that way you can still play the vanilla game or repair mistakes easily without the need to uninstall and reinstall the whole game.
You know how to copy a folder, though, don´t you ?