I'll reproduce the solution Cavey offered, modified a bit so that it'll work for other maps
1. Locate your usermaps folder (default: "C:\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare\usermaps")
2. Locate the folder "mp_mapname" (i.e. mp_isla_volcan) within the usermaps folder. Open it, and delete the files inside. If the folder does not exist, create it.
3. Go to the XI Redirect. Locate the folder for the map you're having trouble with, open it, download the files inside, and place them in the folder you created/emptied.
This should solve any corrupt fastfile issues.