Booting Linux

[Michael & Ulli Hanley]

I just downloaded Ubuntu 7.04 desktop-i386iso onto my laptop hard disk.
I confirmed the integrity of the file with WinMD5Sum.
I burned to a CD using HP Solution center to get to the Windows burn wizard.
I tried opening the file on my hard disk, but message said Windows 
cannot open
I then put the CD into an older computer and tried booting it there.
Even though the setup was modified to put the Cd drive ahead of the hard 
disk and the computer turned off and on, the computer persisted in 
booting up Windows instead.
I could find nothing in the Ubuntu documentation about booting up.
What am I missing?