Live CD/DVD Doesn't Work?!?!

[inertkinesis at verizon.net]

This is my first time on this mailing list or any mailing list so bear with me if I do something too n00bish.

I'm interested in installing Ubuntu on an external hard drive for use as a second OS, but I want to give it a try first, using the Live CD/DVD. I have a computer with an AMD Turion 64 Mobile CPU (ML-44). I've downloaded and burned both the CD and DVD, both of the AMD-64 type. When I boot from them and press enter for the item at the top of the choices list, I get the Ubuntu logo, the progress bar, and the nice brown text underneath. Now, two different things happen, depending on whether I boot from the CD or DVD:

On the CD, it goes through a bunch of stuff, and then the brown text/logo/progress bar screen disappears and I get a blank screen with a command line cursor blinking in the upper left corner. After a few seconds it stops blinking and then nothing happens. Pressing the power button on my computer at this point does nothing.

On the DVD, it gets to a certain point in the loading process and all of a sudden the logo and progress bar are gone and I'm looking at generic gray-on-black system text. But things are still continuing to load. It gets all the way to the end and it says something like "Loading Kernel" and it gives me a cursor. I can type, press enter, etc, but nothing happens when I do any of that. If I press the power button on my computer, Ubuntu unloads everything and shuts down, ejecting the disk and asking me to press ENTER.

So, what is going on here? Am I doing something wrong? Can anybody walk me through it or suggest some alternate things to try?