Again, your Processor is the bottleneck, for this game you will not be able to run it because COD 4 requires a 2.4 Ghz processor, you may be able to play but it will be quite laggy. Do yourself a favor and grab a new CPU and that rig should be fine.
.2GHz won't stop him from PLAYING IT but to enjoy it at decent frame rates is a different story. I would suggest downloading a demo and watching task manager. Look at your cpu usage and ram during gameplay. If it gets near 100% on either and you're lagging non stop then you need to upgrade.
You might be able to play on lowest with little AA and AAF. Since you Ram and Cpu are bottlenecks, you might have to upgrade.... Try Krone6's idea, just download the demo, and if your happy, then your happy. But as far as frames go, if your used to playing on a console, your frames wont be nearly that good with your specs.