HP and Prisonor of Azkaban.

Hello guys.
My problem is that I cant play this game full screen and grafix are not
correct.
My system meets all minimum sys requirements.
My sys specs

Gigabyte motherboard with Onboard Intel 82845 Grafix Card.
256 MB DDR Ram.
Problem is that 8 MB is shared,
So pc says it 248 MB.
Games detects it 248 MB. So starts in Software mode. Is there a solution.