Have you run all the windows updates? If so, you may need
to go to the Intel video control and use the advanced settings
and adjust gamma, brightness, and contrast.
Also, have you tried the keyboard display brightness adjust key?
Saw some thing similar after installing Win8 on my 2011 13" MBP.
All software adjustments did not give any desirable results. Then hit
the display brightness key on the keyboard, and all was fine. It was
odd but some how it seemed the hardware brightness control via the
keyboard does not interact with the software when running Win8. Not
sure if this is an issue with the driver or if it is by design.
Windows has a built-in feature that auto-adjusts the brightness of your screen based on the MBP's light sensor. This feature automatically sets the brightness fairly low, so if you want to turn it off, just hit the start button and search for "adjust screen brightness automatically" under settings and hit the button to turn it off.
I know this is an old thread, but I wanted to share with others who might come across this that I had the problem of a dim screen on Windows thru bootcamp as well, and Wildebeest50's answer solved my problem! Thank you.
Using the Metro menu I went to Start -->
Searched for "Settings" ---->
Clicked on "Settings" on the right hand column bar --->
A whole list of settings pop up, and I clicked on "Adjust my screen brightness automatically" --->
I was taken to "PC Settings", and under "General", I found "Screen" underneath "Time", "App Switching", and "Spelling". --->
I slid the option for automatically adjusting screen brightness to OFF