Got a solution, I run Vista Home Premium x64
I thought it was too easy, but I gave it a shot.
I added tremulous to my games section (just drag the tremulous executable to the window and it will create a shortcut for it) Then edit the shortcut and turn on compatibility mode for Windows XP. After that I never got the error again and the slider worked perfectly for adjusting the brightness.
I'm using the latest nvidia drivers (100.65)
Good Luck! Now back to playing trem :3
EDIT: OH, forgot to mention on the shortcut, make sure Start In: has the correct path in it, in my case that would be
"C:\Program Files (x86)\Tremulous"