glread and draw Problem

Hi everyone.

i have a little problem with my glReadPixels and glDrawPixels Function.

i wrote a program on windows xp with vc++ .net that does some changes on
the image by reading the data
through glReadPixels into a 3dimensional GLubyte array, changing the data,
and write it back to the gpu via glDrawPixels.

That works fine on my computer with a radeon 9800 pro, but not on a computer
with a geforce 7600 gt. On this computer, only the image gets very dark but
nothing else happens.

Has anybody heard of this problem yet?