I have been on Ubuntu for a year, but I am certainly feeling like a total beginner today.

I went from Ubuntu 7.4 to 7.10 via the upgrade process. That was very easy - one click and it was done.

To move to 8.4 I thought I would do a fresh install. So I downloaded the .iso and then burned it onto a CD-R. My Ubuntu PC is set to boot to CD first on start-up. So I booted to it and nothing happened - it just checked the CD and then installed 7.10 as per normal from the hard disc. I tried booting to the CD on my alternate (Windows XP) PC and the same thing happened - no installer came up. Clicking on the .iso file achieves nothing also.

I know that I am probably doing something very basic wrong here. What?