My strong advice is pick up a good computer from a good store with the best warranty. I prodded myself into a 3 year warranty [3 years ago]. I took it in with one month to go for some final maintenance and to fix a faulty dvd drive and they gave me an updated/upgraded/brand spanking new dvd drive for free. And blew out a bucket of dust from inside the pc case.
Also if you meant 'disable windows automatic updates' -you don't want to do that. Instead set the option to 'alert you when Updates are Available' that way you can download and install the sometimes critical updates on your own time. - To get to update notification options [if you are using xp] right click your My Computer icon, select Properties, then the tab 'Automatic Updates'.
Another thing I do, is put my games in their own partition on the hard drive. My CoD is in its very own partition; its very useful when you want to defrag the partition [clean up scattered files - and make gameplay smoother].... you save alot of time -especially with a HD as big as yours, I'd suggest that option- I don't know if Partition Magic is still around, but do some research on it- it's a reputable software tool makes the partitioning process incredibly easy.
Two game cards is an option- but not necessary in my opinion- you have what's necessary already regarding ram and game card to play a strong game.... I'm sure its other issues that are the source of the lag issues.
Try the CoD game option to auto resize at 800x600 screen -no need to change your actual normally used screen size.... and lower the sound setting, and remove a few effects you think you don't notice in the game.
Visually 'maybe' it bothers you ... but perhaps you won't notice it in place of playing a satisfying faster/better game.