I've been having this problem for the longest time with counter strike source where the AI lags and everything else moves at normal speeds, the AI moves a little slow and Jitters a lot. With bio shock, first a few months ago i got the demo and the physics and the AI that used more physics (like the helicopter bots) lagged and the physics in general lagged. I didn't worry because i thought it was just the demo. I got the full game (non steam version) a few days ago and i have the same problem, it's like in css where only certain things lag. in both games my movement is done at a normal speed but when i interact with certain objects they lag while some don't. I'm thinking maybe it's one of the process i am running with the start of windows, i run three security apps (clam av, avast and spycather) plus free ram xp (I know it's a lot but they don't slow down other games or demos like UT3 and COD4 and hl2)
my specs are: intel core 2 duo 2.4 ghz, nvidia 8800 gts 640 mb, 2 gig ram and xp sp3 release candidate (i was having the problem in sp2 also)
thank you in advance!
EDIT: i tried shutting down the apps that would allow me to: clam av, free ram and spy catcher (avast wouldn't let me shut it all the way off, but i turned off all the services). and the game (bioshock) lagged a little less but not a ton.