Intel C2D e6230 @ 3.0GHz
And yes Windows Vista Home Premium 64-bit Edition (Never understand why people slate it so much - I've had absolutely no major problems with it and it's a joy to use) What OS do you use?
I've never had any major problems with CoD 4 myself (apart from when using my Medusa 5.1 phones they very occasionally seem to randomly lose power, volume and 'richness' of sound drops until i restart the game) and I've spent over 2 days solid on that game.
Since you say you don't have any problems with other games I think it's fair to say that it's purely a software problem. In that case you need to ensure:
You have all of the correct and latest drivers for your hardware
Have patched CoD 4 to it's latest version (1.5 I believe)
Apart from that I would really know what else to suggest apart from a bad install of the game perhaps. Very important note - I've heard that if you don't backup your online profile information when you do a reinstall you'll lose everything you've worked so hard for while playing online (Rank, weapons, perks etc.) So make sure to backup!