I have upgraded to 12.04 but when I boot the PC I still get the purple 11.10 screen with the white dots.
How do I change this?
I am running on an AMD64 processor and have an Nvidia GE 9300 graphics card
Both 11.10 and 12.04 comes with LightDM.
Purple screen and white dots you are referencing to is actually LightDM welcome screen. It hasn't visually changed between 11.10 and 12.04.
11.10 LightDM welcome screen:
12.04 LightDM welcome screen: