iMac display — can't change screen brightness on bootcamp with Windows 7

Question

I can't change iMac (2011) display brightness on bootcamp with Windows 7. When I press F1 it shows icon and even fill the bar, but nothing happen to display.

What have I tried:

• Reinstalling support files
• Installing fresh drivers from ATI site.
• Changing screen brigtness in windows "Power Options".

Answer 1

I solved my problem, but I don't actually know what solved it:

• I rebooted manualy without any third-party software like BootChamp.
• I was pressing Alt (option) key when booted.