I just built a new PC for Left4DEad with an AMD Athlon 64 X2 5200+ processor. I loaded XP Pro SP2 and immediately encountered a severe stuttering problem playing L4D.
L4D was completely unplayable ...
So I installed the AMD Dual Core Optimizer and hoped that this would fix the issue - but alas it did not.
The solution that I found was to disable Multicore Rendering in the Video Options as follows:
Let's hope that there is a permanent fix from Valve soon!