First, just a note about re-installations. With Ubuntu, a circumstance that will require a complete re-installation of your system is very rare; perhaps mechanical failure of the hard disk would be the most common case.
Many common operating systems may require a complete installation to be performed when critical failures take place or for upgrades to new OS versions. Even if a completely new installation isn't required, often the programs you use must be re-installed to operate properly in the new OS.
Under Ubuntu, it is much more likely that your OS can be repaired rather than replaced if things go wrong. Upgrades never require a wholesale installation; you can always upgrade in-place. And the programs are almost always compatible with successive OS releases. If a new program version requires newer supporting software, the Ubuntu packaging system ensures that all the necessary software is automatically identified and installed. The point is, much effort has been put into avoiding the need for re-installation, so think of it as your very last option. The installer is not designed to re-install over an existing system.
Here's a road map for the steps you will take during the installation process.
Back up any existing data or documents on the hard disk where you plan to install.
Gather information about your computer and any needed documentation, before starting the installation.
Locate and/or download the installer software and any specialized driver or firmware files your machine requires.
Set up boot media such as CDs/DVDs/USB sticks or provide a network boot infrastructure from which the installer can be booted.
Boot the installation system.
Select the installation language.
Activate the ethernet network connection, if available.
If necessary, resize existing partitions on your target harddisk to make space for the installation.
Create and mount the partitions on which Ubuntu will be installed.
Watch the automatic download/install/setup of the base system.
Install a boot loader which can start up Ubuntu and/or your existing system.
Load the newly installed system for the first time.
If you have problems during the installation, it helps to know which packages are involved in which steps. Introducing the leading software actors in this installation drama:
The installer software,
the primary concern of this manual. It detects hardware and loads
appropriate drivers, uses
dhcp-client to set up the
network connection, runs
debootstrap to install
the base system packages, and runs
tasksel to allow
you to install certain additional software.
Many more actors play smaller parts in this process,
debian-installer has completed its task when
you load the new system for the first time.
To tune the system to your needs,
tasksel allows you
to choose to install various predefined bundles of software like a Web
server or a Desktop environment.
Just be aware that the X Window System is completely separate from
debian-installer, and in fact is much more
complicated. Troubleshooting of the X Window
System is not within the scope of this manual.