First you need to burn it to CD as a CD image.
Then change boot order to read your CD or DVD drive BEFORE your harddrive - do this in the BIOS.
Insert CD, and boot machine.
Choose the first option, I think it's Install or Run.
You can now run Ubuntu from the CD to try it out.
You will see an installer on the Desktop - do NOT click on this unless you want to install Ubuntu to your hard drive.
I suggest you try running it from the CD for a week to see how you like it.
Changing the boot sequence will not affect your 2000 install, it simply means that you get the option to boot from CD or DVD as well as your hard drive.
Also, lots of other Linux distributions have CD's and DVD's where you can try the OS out before installing.
I would highly recommend you give the openSUSE 10.2 Live DVD a try as well.
http://download.opensuse.org/
Note: The SUSE Live DVD does NOT include an installer, you must download a separate ISO for this.