Unfortunately, you'll never have complete control of the backlight in Windows like you do in OS X. You can try to update your Boot Camp drivers to 3.1 but you'll still have problems. From my experience, 3.1 drivers will allow you to completely turn off the backlight but it will randomly turn on/off by itself with no change in ambient lighting. There are some rumors out there that Apple is working on backlighting but I'm not holding my breath, it took them while to finally get the trackpad problem fixed.