Transparent color isn't transparent.

I have some problems with Delphi and some screen-drivers.
I've made an application with buttons having bitmaps. standard Delphi's
When I run this program on my computer, all is correct.
When I take the same program and run it on another computer. I've
problems because the transparent color is displayed.
When I change the resolution of this computer (from 16bits to 256colors
for example). All is correct, I've a good transparency.
I've this problem on some computers, and not on others. It seems to be
an incompatibility between Delphi's transparency and some video-drivers.

Has someone have a solutions? How can I do for display this bitmaps
correctly everywhere?