Solution:
Alpha blending uses a fourth color component which is not displayed but which corresponds to the opacity of a surface. Alpha blending controls the amount of color of a pixel in the source surface to be blended with a pixel in the destination surface.

This applies to:

Intel® 82810 Graphics and Memory Controller Hub (GMCH) Intel® 82815 Graphics Controller Intel® 82865G Graphics and Memory Controller Hub (GMCH) Mobile Intel® 915GM/GMS, 910GML Express Chipset Family