Bootcamp issue!

Hi, I installed windows 7 ultimate in bootcamp, and it seemed to run fine after a few hiccups. Though last night after playing for a fairly decent time on shogun 2, it froze on the shutting down page, so i then switched it off.
Now whenever I try to log into windows, it starts loading up: screen goes black with the underscroll flashing in top left hand corner, but then it just restarts my mac...
Anyone know how to fix this??