Here, we've created a twelve-step visual guide to getting Ubuntu up and running with all the most common bases covered. There's more than one way to touch each one of those bases, so this guide mentions some of the common options available for a newly created Ubuntu installation.
1. Before You Get Started
If you're an existing Windows user looking to make the jump to Linux, don't just throw in the CD and hit the buttons. Plan ahead. You might want to use Wubi to install Ubuntu on your existing Windows partition -- it's non-destructive, doesn't require you to change partitions and uses the existing Windows boot loader.
Those who want to resize an existing Windows partition can use the native resize utility in Vista -- it's in Control Panel\Administrative Tools\Computer Management\Disk Management .
If you're on XP, you can use the partition-management tool that comes up in the Ubuntu installer, or use GParted from most any live Linux CD.
In either case, be careful -- don't mess with a partition unless you've already backed up everything you can't replace, either file-by-file or with a disk imaging utility like DriveImage XML.
The best possible scenario: install to space you've already cleared out, like a newly-created partition, or use an entirely new drive for Ubuntu. Tip: Don't format the partition or drive just yet; that way it'll be easier to single out in the Ubuntu setup menus.
Finally, before you start the installation, remove anything that doesn't absolutely need to be there for the setup -- mainly, extra peripherals such as scanners, printers, and so on. This keeps the number of variables to a minimum during the installation, and you can always add them back in one at a time later on. (I've found this is sound advice for both Windows and Linux installations.)