The gpu framebuffer still has the background image stored in it, or sometimes other things. It is normal to show the framebuffer's contents, but it should clear out, and be replaced long before human eye could see the scramble. Due to a fault in driver, some graphics cards make the whole system freeze and the framebuffer never clears out. Usually the framebuffer clearing would be a hardly noticeable flicker just before something appears on the screen.
I have a nvidia graphics card in my laptop and I suspect that the issue is due to the standard driver from the cd. If you manage to boot in vesa mode (not using the standard driver), it will start. Then, to get graphics acceleration, you should download linux driver from the nvidia page and then install the driver (requires kernel-source amongst other things). After that it will be all fine.
I am aware that i am months late, but someone might still need the info.