was the old display adapter onboard and did you uninstall or disable via device manager?try this.
restart your computer and enter your bios (will say on screen which key to press at post)
look in advanced chipset or similar bios menu for "shared memory" and disable.also look for "agp boot from" or similar and set to auto or agp.
try booting into safe mode(keep tapping F8 as the computer starts and select it)try lowering your display resolution to 800x600 and then restart and uninstall the ati control panel via add remove(dont restart)then the driver and restart.download the latest cat 4.1 driver click here
try driver cleaner click here