@LambOfGod - Thanks, however I do those maintenance steps all the time and have made a load of registry hacks to improve Vista's performance. As a result, my 'day-to-day browsing' performance is even better than my desktop's.
Strangely enough, I completely overlooked my video cards drivers, and DXDIAG revealed I was running on May, 2007 drivers! I upgraded my drivers and the games seem to be performing as they should now - COD4 works smoothly with everything on high (except for AA of course), and COD5 is a little bit slower but still playable. Left 4 Dead had to have its shaders turned down, but now things are working great. I feel silly for not thinking about the drivers.
Thanks for your help, guys! I really appreciate it.
-Rhys