I have a similar problem with my new HP TouchSmart 300 series PC. Even though I set the desired brightness level in the Power options, my display jumps back to 100% brightness after power-on, restart, waking up from sleep mode or returning from screensaver mode.
Any of these latter processes appear to negate the manual settings. I'm still trying to understand, whether it's a a Windows 7 issue, or a PC specific hardware/software issue. Any suggestions will be appreciated.