Dell Laptop Monitor keeps dimming on its own

Systems Latitude E4200 Latitude E4300 Latitude E5400 Latitude E5500 Latitude E6400 Latitude E6400 ATG Latitude E6500 Precision M2400 Precision M4400 Precision M6400 

You can enable or disable the Ambient Light Sensor

The ambient light sensor is located on the front of the computer display. The sensor detects available environmental light and automatically increases or decreases the display backlighting to compensate for low-light and high-light environments.

You can enable or disable the ambient light sensor feature by pressing the <Fn> and left-arrow key combination.

Do not cover the ambient light sensor with any adhesive labels. If covered up, the sensor when enabled automatically sets the display brightness to the minimum level.

The ambient light sensor adjusts the display backlighting on your portable computer only. It does not control the brightness on any external monitors or projectors.

There is a software for Vista to Manage if you want to use it.  Dell Ambient Light Sensor

http://www.dell.com/support/drivers/us/en/555/DriverDetails/DriverFileFormats?c=us&l=en&s=biz&cs=555&DriverId=R191268