Those complaining of slowdown on high-end systems, or that the game runs smooth for a while and then starts juddering, are experiencing the notorious "Steam Stutter".
Has nothing to do with hardware slowdown, it's a weird characteristic glitch on Steam. If you look on Google you can find info on how to edit the game's config files to alleviate some of the problems.
There's also a Security Update fix for XP that b0rks FEAR and Dark Messiah, stops them from working after they try to access the net. You need to download a second security update manually from Windows Update to get it working again.