I typed "Ubuntu download" into my browser, and up came the link to download the CD image. Copy that to CD, then install.
It takes some time, but everything works well. In this case, "everything" is a Dell 4550 with a 80 GB hard drive that was being thrown away.
The internet works, documents work, all the ports and drives work - I'm a happy camper.
Now to go and do something with the technology...