Windows keeps rebooting??

Hello,

I'm running on 10.8.2 and have installed Windows 7 home 64 bit using boot camp. I also own a version of parallels, which works great. When I'm running the mac I can access my windows installation perfectly fine but for some reason when I restart and try to boot directly into windows by holding the option key it just gives the little white dash in the top left corner and keeps rebooting. Why would I be having such a problem if it runs fine when I us parallels to boot it up?

Thanks