Nope, didn't even have anti-virus installed for weeks and it did this. This is by the way 100% same stuttering as most people had in America's Army 3 Pipeline map (uses UE3 engine too, and was a complex object filled multi-story building like in Station) but they fixed it in some update.
Nice post. My rig is similar to yours and I have experienced first hand the micro-stutter on station that you referred to in your original post, other maps such as Pavlov's house seem to develop the same prob over time while playing.
I have just had a thought that perhaps the issue lies with physx in some way. I have messed around with HOS settings at the cfg level, resulting in very little difference in performance/gaming. The only setting that cannot be disabled is the Physx (not sure if the game would allow disabling of the physics engine without ruining the game).
Anyhow if push comes to shove I might even reinstall xp and see if the prob still exists.
Will keep peeps posted on my endeavours..
Cheers Dan