If you installed the Game With Steam or the Retail DVD, you can play the multiplayer version without need to download the full installer, you have these options:
1) Keep the Full Game as is. If you do, you will need to download the FEAR Launcher to join Servers. This has the disadvantage that you will need to wait about 2 min to join a server with the launcher (the ingame server list won’t work). Also you need the new CDKey you got in this site ( in the right sidebar) to play online. You can change It with the launcher too.
2) Install the patch only; is less than 10 MB download, and will Fix all ingame errors caused by the UFF Event. This has the disadvantage that you can’t play single player; you can restore backup files to enable single player again, but you need some knowledge on moving game files.
3) Keep single player as Is, and install FEAR Combat, from this site. This way you have both, Single player and Multiplayer.
Steam doesn’t provide a CDKey when you buy the game, and old original Keys provided with the Retail DVD’s don’t work anymore (after the UFF Event). So you need to register to get a new CD Key.