I recently bought a Toshiba laptop, dropped Vista and setup XP.
Everything is working fine except for the fact that I cannot decrease
the brightness of the screen using FN + F6. I've looked on Toshiba's
website for keyboard drivers but haven't found anything. I've searched
Google to see if anyone else has encountered this problem and haven't
found anything.

Any help would be appreciated.

G'day,
I am having the same problem as yourself. I've read what seems like a possible solution. Here it is

On some notebook models there is difference between Toshiba hotkey utility and Toshiba hotkey utility for display devices and those are two different utilities. Because of that I have recommended to install the hotkey utility alone. I didn’t know that it is not available for Tecra M2 separately.

The problem is that functionality of FN keys depends on right installation order and the most important is “Common modules” and must be preinstalled at first, before you start to install any other Toshiba designed tool or utility. It is not easy to say why this happen in your case but try to reinstall Common modules again. I don’t know if this will help but you can try.

This worked for the fellow seeking a solution. So, maybe...

I have found where to get this 'Common modules' from. Just from the Toshiba site where there is a whole host of new drives.