If you want to play the F.E.A.R.in multiplayer mode for free, read this guide.
Install game and the update. Replace the original exe with the exe from the links above. Now go to this address and register for a free serial (is for FEAR Combat game but we can use this to FEAR MP).
When finish, you will receive a free valid serial number. Go to your email and write down the serial.
Run the game. Select Multiplayer. Normally, when you try to join a server (with any invalid key), you will receive this message.
To play the game with the provided serial, click on client settings. Now in SN field, type the code (you received). In here, enable the PunkBuster , because some servers require it.
Now press ok to go back. Click on receive servers, click on any and play.