Could you provide more information about your exact Mac model and what Windows version you use?
When I used Windows 5.1 and 6.1 via Boot Camp to play some games, my Mac got hot, but that was due to CPU and GPU intensive games. Otherwise the temps were okay on my 2009 MBP and 2007 iMac.
What is too hot?
Have you any kind of monitoring application installed and used it?
Maybe you could use SMC Fan Control to set the fan speed in Mac OS X and then restart into Windows.
Have you taken a look at the Task Manager and looked at the CPU activity and what process(es) might hog it down (thus creating more heat)?