i got the same error..
if i wanted 2 choose openGL mode in halflife (my direct3d sux) and started a game, a screen came up and said 'your computer doesn't supprt the selected mode'
But i've now downloaded the newest drivers for my videocard(nvidea tnt2) and now it works again.
i've only had this problem with windows XP. maybe it's the same with you..
:S