wierd rendering problem in Windows

I got an HP t870.no desktop computer, with ATI Radeon XPress 200P chipset and a Radeon X300 graphic card installed - all stock components.

I was just trying to replace the X300 with a Nvidia 7900GT card. I do not see any rendering problems while standard VGA drivers running. But after installing nvidia drivers, rendering problems occures in explorer windows and on the Start-menu. No errors in 3D-gaming, so I assume it has to be something with XP or an compabillity error, though I do not think it is likely..

I've tried older and the newest nvidia drivers - it gives the same result.

Any HW experts wanting to give a shot?  

My error is shown below: (of Start-menu)