I found and downloaded this:
CoD4MW-1.6-1.7-PatchSetup.exe
I won't provide the exact link here. You will have to google it and find your own. When you open COD4, it tells you your version at the bottom right. You want it to start with 1.7....... something the only thing that matters is the first 2 numbers. You are probably at 1.6 right now. Go confirm that and download the patch. Have fun in the BFM server!