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This applies to both Call of Duty 4: Modern Warfare and Call of Duty: World at War, and will be posted on both boards as such. Also, I assume that you meet the system requirements for the game and you know your way around your PC as well.
Now, personally, I'm running Vista 64-bit and have both games running through Valve's Steam service, but this should work for anybody else, as well.
Here are the problems I was faced with.
Call of Duty: World at War error and crash during gameplay: Unhandled exception caught
Call of Duty 4: Modern Warfare has stopped working (crash when I tried to load CoD4's multiplayer).
Apparently, this has to do with the sound hardware on your computer. My sound card is a Realtek High Definition Audio. There are multiple ways to fix this, regardless of your hardware.
1. Update your sound card's drivers.
Self-explanatory. Windows told me that my sound card's drivers were up to date when I tried to update them automatically, but when I checked Realtek's website, I found out they weren't. The only way to tell is by going to your sound card manufacturer's website and checking for updates manually.
2. Turn off any modems you're not using.
Go to your Control Panel and Device Manager, then under "Sound, video and game controllers", disable any modems you see that you're not using. If you're like me, you don't have any modems at all in your computer, so keep reading.
3. Set the game to Compatibility Mode.
Find the .exe for CoDWaW or CoD4MW and right click it and go to Properties. Under the Compatibility tab, check the "Run this program in compatibility mode for:" box and then select "Windows XP Service Pack 2". I assume this only applies to Vista users.
4. Make sure your sound format isn't set too high.
Go to the Control Panel and the Sound properties. Find whichever device is your default one (it has a green checkmark next to it) and right click it and go to Properties. Go to the Advanced tab. Look for the Default Format drop-down menu. Set the value to 48000 Hz or lower (bit rate doesn't matter) and apply the settings.
5. Plug something into the microphone jack on your sound card.
Odd as it may sound, apparently this has worked for some people. It doesn't have to be a microphone, it can just be a pair of headphones or anything that will fit into the jack. Don't plug anything that's powered by batteries or otherwise into the jack, such as an iPod through an output cable, because it can possibly damage the sound card. A crappy pair of earphones will do the trick.
6. Enable Stereo Mix
This is what worked for me, and it's a really odd one. Go to your Control Panel and the Sound tool again. Click on the Recording tab. Right-click anywhere in the Playback Devices box and click "Show Disabled Devices". From there, enable "Stereo Mix".
If you're still having either of these problems after doing any of the above, post them here and I will respond to them with whatever help I can. Otherwise, tell me if these solutions worked for you. Good luck, and happy gaming.