Software to adjust display brightness on Windows XP laptops

I have a lot of great experience with this software PowerStrip

"Why are you adjusting brightness? If it's because, as the day goes by, lighting changes and your screen becomes difficult to see, try f.lux. It has Windows, and Linux versions, and is brilliant software. Completely unnoticable when it's on, but when it's off, you know what you're missing.

F.lux changes the colour warmth of your screen to match the sun, which is pretty nice. It's all very slick, no messing around with configuration every day to keep it in sync, it uses your latitude to work it out."