Hi and welcome to LQ.org. When you boot with your desktop CD you have an option to start normal, harddisk and safe graphics mode (or something like that). Choose the safe graphics mode. I've a geforce 6600GT and I got a similar problem as yours.
The drawback is that you won't be able to choose many resolutions. But don't you worry, once Ubuntu is properly installed, with one command, you will get nvidia drivers installed and the thing will fly