Is there an application for Windows7 (in bootcamp) to adjust the level of brightness of screen of iMac?
You do not need any software to do this. The Boot Camp drivers from Apple supports changing the brightness of the screen (using the keyboard modifier keys).

The drivers are already included in your Mac OS X DVD for installing Boot Camp Utilities. Otherwise, you can download the installation files using Boot Camp Assistant at /Applications/Utilities.

See here for more details.