Hi,
got the same problems on my system if change the renderer to dx9.
I thought if DX10 stutters i change to dx9 but not in this game.
Strongly recommen you to use Win7 and dx10 renders. XP support is cut off soon anyway and on work in store we dont sell any xp since 1 year
ps using 270 nvidia drivers is a good idea. it boosts my fps quite good on gtx460