For a dual booting machine: If you currently have Windows XP/Win 7 on your machine and it occupies your whole Hard disk then you would first need to create space for the 2nd OS. You have to partition the hard disk.
1. Backup your important data
2. Download
Gparted. All instructions are on the Gparted website.
3. Burn the downloaded Gparted .iso image file to a CD/DVD.
4. Boot from the newly created CD/DVD
5. Resize your HD (hard disk) partition using Gparted, making the C:\ partition smaller and apply. Gparted will now resize your partition on-the-fly. Remember to resize so that have the blank space is on the right hand side of the screen, the end of your hard disk. Windows 7 needs at least 20Gb.
6. A restart might be required for Gparted to complete process.
7. Restart machine and boot from the Opertaing System disk you want to install.
8. On the install screen you would you have to select the disk/partition you want to install to. Select the 'unallocated' space. Windows will now install and when finished would have automatically created a dual boot.
My advice is rather to use the previous poster's advice. Install the free Virtualbox, install whatever OS you want to test. That way you don't have to mess with sizing discs, formatting etc.
All the best !