This page describes how to set up your computer in order to dual boot Ubuntu and Windows.
Back Up Your Data
Although this may seem obvious, it is important to backup your data files to an external backup medium before attempting a dual-boot install (or any other hard drive manipulation), in case your hard drive becomes corrupted during the process. External hard drives, USB flash drives, and multiple DVDs or CDs are useful as external backup media.
Have a Windows recovery CD/DVD available
Many computer manufacturers that pre-install Windows provide a Windows recovery/re-installation CD or DVD with the computer. However, many companies no longer ship a physical disk but instead create a hidden partition on the hard drive in which the recovery-disk information is stored. A utility is then usually provided which allows the user to burn a recovery/re-installation CD or DVD from it. If you are buying a new computer and intent on dual-booting, make sure you have (or can make) a physical Windows recovery/re-installation CD or DVD. If neither a CD/DVD nor a recovery partition/burning utility is provided by your computer manufacturer, you may need to telephone your vendor and ask for a CD or DVD (to which you are normally entitled under the Windows EULA).
It is very important to have such a physical recovery/re-installation CD/DVD prior to changing your hard drive, in case your hard drive becomes corrupted in the process (and the recovery partition thereby becomes unavailable for use).
Once you have created a physical backup disk from a restore-image partition on the hard-drive, the restore-image partition can either be removed or left in place. Ubuntu can be installed with it intact without problems.