Installation: Download patches 1.4, 1.4-1.5 and 1.6. Install the patches in order (the patch installer should be able to find your CoD4 game folder). If you have a non-evaluation copy of CoD4 you are now done.

If you are using an "evaluation" copy of CoD4, you also need to do the following:

Download the CoD4 SP_MP Client Patch, and unzip it to your desktop. This patch contains two files (iw3sp.exe and iw3mp.exe). Drag these two files into your CoD4/main directory replacing the files with the same name that are already there and you are done.