Bugs in drivers can cause screen savers not to start, glitches in 3D as well as regular applications, or even the computer to crash. Drivers are a big deal.
I am guessing you have the "Automatic" or "Windows Update" drivers which usually are NOT optimized for 3D. I'd head to ATI's website and download the latest for your 7500 card.
P.S. I merged your two threads since it is the same issue. Please try different drivers.
The latest that ATI ever
released is version 6.11.