hard disk activity whilst gaming problem

If you only have 2 or 3GB you are probably running out of RAM and hitting swap on your hard drive. I'd recommend having at least 4-8GB of ram on vista if you play games but not everyone can or wants to do that. One quick way to keep some memory freed up in vista is disabling the SuperFetch service. That caches frequently used programs into RAM so they open faster later. I can "maybe" see the use for that on laptops but most desktop hard drives are fast enough now that it is unnecissary.

Right click on Computer and click manage
Expand the Services and Applications menu and click Services
Scroll through the list until you find Superfetch and double click it
Under Service Status click Stop
Change the startup type to Disabled

Even with 4GB SuperFetch could potentially be a problem. The longer your system is on the more it caches. Check Task Manager before you start a game and see what the Memory usage is at. If it is around 2GB+ you can expect to hit swap on memory intensive games like COD4. Make sure you have as few things running in the background as possible. Closing chat software, sharing software, or any other things that might hang out in your system tray will help.