Some installs have unique problems, this is not always your fault here are some things MIB88 recommends you try: I followed the above guide to the letter, and it still wont work! Trust me you cannot be to careful here, this is where 90% of people go wrong. I would personally un-mark, save, and the re-mark the folder as read only, then I would check the individual files. This is the most crucial step, a hint of paranoia is an advantage.

If the box is already ticked don't trust your computer. Set the entire proto folder to read-only, the proto folder is under Fallout2/data/proto you simply need to right click on the folder and tick the read only box.Fallout2/data/data, yes two data folders are supposed to be there, do not concern your self with the second unless your specific mod read me says so. Install the mod you downloaded, most of the time you simply copy and paste the files into your FO2 directory, some people get confused with the directory structure i.e.Check your FO2 folder, for a file called patch000.dat, if you have it delete or rename it.Install the version of Fallout 2 that takes up the largest amount of space i.e.Generally mods contain a 'readme' file with a guide, I would recommend checking that out, before proceeding with this general guide, also before you attempt to install a mod, make sure FO2 works fine vanilla:

Fallout 3 mods installieren steam