Introduction
These pages describe how to set-up your computer in order to dual-boot (or multi-boot) Ubuntu and another Operating System(s), such as Windows or even another install of Ubuntu.
Back Up Your Data
First, of course, backup your data files and settings to an external backup medium before attempting any kind of install (or any other hard-drive manipulation). Accidents do happen, especially if you have not taken reasonable precautions beforehand. External hard drives, USB flash drives, and multiple DVDs or CDs are useful as external backup media.
One of the reasons people find linux easier to work with is that ALL the data and settings of all the users on a system are all contained within one easy to find folder. With some other operating systems (notably Windows) there are often stray parts, such as photo-albums, that are scattered elsewhere within their folders (hint look in "Application Data" folders both inside and outside the users folders).
Have a Recovery Cd for the existing Operating System(s)
Many Operating Systems allow their recovery system to be on a different media such as Usb-stick or Dvd and some even have a recovery or repair system on their installer Cd.
For the Windows one see here. Most linux systems can be repaired from most other linux systems and/or their recovery or installer systems are easy to download given a working system such as a LiveCD.
A very tiny linux distro often found useful if Ubuntu's download is too much for a system is sliTaz although many people find Trinity Rescue Kit quite useful for fixing problems with Windows. Trinity is command-line only, which is why i prefer sliTaz for most simple tasks if Ubuntu is unavailable for some reason. Visit the DistroWatch home-page for the top 100 list of most popular distros and notice that Ubuntu is top of the list.
Some Cases
General Case
In general whichever operating system you are going to dual-boot (or multi-boot) with the procedure is very much the same. There are a lot of guides on how to dual-boot with Windows because this is likely to be a persons first attempt at setting up a dual-boot system. It is also the most difficult and so Ubuntu has an automatic system built into the installer.
With Windows
Please see the sub-page DualBoot/Windows. There is quite a lot of detail but mostly it is how to deal with unusual problems. In general just follow the obvious section in the "Partitioning Section" of the Ubuntu installer.