Post #1: Backup & try before installing a new system (this post)
Post #2: This post is about UEFI (this link)
Post #3: mini.iso, minimal install, netboot iso (this link)
Backup before installing a new system and create a regular habit to backup your personal data
Editing partitions and installing operating systems are risky operations, and I recommend that you backup at least all your personal data (documents, pictures, ...) before you start. In general, it is a good idea to have a regular habit to backup the personal data. You never know, when something bad will happen to the computer, and you will need to restore the backed up data.
Try Ubuntu (and the community Ubuntu flavours) before installing
It is a good idea to try Ubuntu (Kubuntu, Lubuntu, Xubuntu, Ubuntu Gnome, Ubuntu Studio, ...) before installing into an internal drive. This can save a lot of trouble compared to installing directly.
This link contains links to all Ubuntu flavours: http://releases.ubuntu.com/
1. Run a live session booted from a CD/DVD/USB drive and select 'Try Ubuntu without installing'
Download a desktop iso file, create a CD/DVD/USB boot drive and reboot the computer from that drive. Check that the download was successful with md5sum.
a. Basic install drive (live drive)
See instructions at this link
with instructions to burn a DVD on Ubuntu, Windows, MacOS
and instructions to create a bootable USB stick (pendrive) on Ubuntu, Windows, MacOS
CD disks can be used to install Lubuntu and Ubuntu mini.iso while the other iso files are too big for CD disks. (But mini.iso does not run a live session, it only installs.)
Find more details for booting from USB at https://help.ubuntu.com/community/In...n/FromUSBStick
Basic install drives (live drives) are the standard tool to try Ubuntu live, and if it works well, to install Ubuntu to an internal drive.
b. Persistent live drive
It is fairly easy to create a persistent live drive with a USB pendrive using the Ubuntu Startup Disk Creator, Unetbootin or some similar dedicated tool as described here. There are detailed instructions at the following links to get persistence with CD/DVD and USB drives.
Such drives are similar to basic live drives, and in addition can save settings, installed programs and data files, but are slower (with the same hardware). The kernel cannot be upgraded.
2. Install (a complete installed system) to a USB stick or pendrive and boot from it.
This way you can find which version and flavour that works best when installed without touching the internal drive. It is also the way to try, if you use an installer which does not offer a live session, for example from the Ubuntu mini.iso or an alternate iso file and Ubuntu Server. You find working mini.iso files for 12.04 LTS (32-bits pae (and non-pae in a subdirectory)) at this link.
It is a two step procedure: Make a CD/DVD/USB boot drive, boot from it and install into a[nother] USB pendrive. Make sure that you install the bootloader into the head of the target pendrive (not into the internal drive or the source pendrive and not into a partition).
This will test a complete installed system (but in a USB pendrive). Boot from this installed system and check what works directly, what needs tweaking (boot options, or drivers for graphics, wifi etc).
It is also a method to create a portable Ubuntu system in a USB drive.
The normal installation method described above works also to create an installed system in a pendrive. See also these links describing different methods to create installed systems in USB sticks or pendrives
for new or middle-aged computers https://help.ubuntu.com/community/In.../UEFI-and-BIOS
for old or middle-aged computers https://wiki.ubuntu.com/Lubuntu/AdvancedMethods
and finally this link for old computers Old hardware brought back to life