Windows 8 Boot Camp Control Panel on MacBook Air

Probably tuning down a lot of the graphics settings of Windows will at least help you unload the GPU.

Or you could try something like CoolBook, which undervolts your CPU and thereby reduces your temperature.

I realize that these answers won't really satisfy you, but I think the driver support under Boot Camp could use some improvement, before a real solution can be given (see nVidia drivers with Multi-GPU)

Or you could resort to installing Windows 7, which at least loads the system less intensive...

I booted into Mac OS by holding down Alt/option key while the computer was booting up.

I also read that holding down C key works, but it did not work for me.

Here is the info from Apple:

Key or key combination  What it does
Option      Display all bootable volumes (Startup Manager)
Shift       Perform Safe Boot (start up in Safe Mode)
C           Start from a bootable disc (DVD, CD)
T           Start in FireWire target disk mode
N           Start from NetBoot server
X           Force Mac OS X startup (if non-Mac OS X startup volumes are present)
Command-V   Start in Verbose Mode
Command-S   Start in Single User Mode