Some guy on that forum.
mpdata information: rank, chanllenges and custom classes are saved here. if you change your keycode, cod4 says "your mpdata is corrupt, resetting stats". when i renamed mpdata.corrupt back to mpdata and used old keycode, my rank, challenges and custom classes came back but the names of the custom classes. they are now called "Custom Class Slot" 1 to 5 and i have to rename them again. i guess the names aren't saved here (?).
One thing I found so far, makes some sense. I'd say go into your profile folder, and your player's name. You'll see mpdata.corrupt in there if you're still getting that corruption error. It should fix it. Or at least lead you on the right track. More info here: http://www.modcod4.com/e107_plugins/forum/forum_viewtopic.php?776But because it is a mod, you will have to check out the mpdata from the mod folder in your profile.C:\Program Files\Activision\Call of Duty 4 - Modern Warfare\players\profiles\ \mods\ftag\