If you are trying to get an old server up and running or having trouble with your current installation, let me know:

The primary triumph of the v1.8 revision is the elimination of exploits. It patches engine-level memory leaks and prevents server-to-client buffer overflows. This ensures that simply joining a public server no longer poses a risk to your personal computer. Restored Master Server List

To fix the issue and join servers again, you must replace the broken 1.8 multiplayer executable ( iw3mp.exe ) with the authentic 1.7 version. Method 1: The 1.7 Executable Replace

Allows servers to utilize more assets, meaning more detailed custom maps and complex script mods without engine crashes. How to Install the CoD4 1.8 Patch

What are you playing on? (Steam, old retail disc, or Mac?)