Ubuntu 12.04 blank screen on boot

I'm trying to run the 12.04 Desktop Live DVD.
The PC boots from the disc OK. It shows a purple screen with an image at the bottom (a keyboard & a man I think). It then changes to another purple screen, with the word UBUNTU in it & 5 flashing dots underneath. After a while, the dots all light up and a few mins later, the screen goes blank & the PC locks up. I have to power it off, nothing else works.
If I use an older disc (the 10.04 desktop disc), there is no problem at all, it boots up perfectly.

PC spec:
Intel® Core™ i7-860 processor 2.80GHz
4GB DDR3 SDRAM
NVIDIA® GeForce® GT 340.

Any help gratefully accepted.