Virtualization software helps you run multiple operating systems on the same computer. For instance, you can use a virtualization tool like Apple Boot Camp or Parallels to run Windows on your Mac computer or you may use the free Virtual PC program from Microsoft to run Linux, DOS or even multiple Windows environments inside your Windows PC.

You may use these programs to try out other operating systems on your computer without disturbing the existing setup. Or, if you have upgraded to a newer OS, you can create a virtual machine of your previous OS and use it to run older programs that aren’t supported in your new OS. Or if you want to run multiple versions of the same software (like Photoshop CS4 and Photoshop CS5), you can install one of them on your main OS and the other inside a virtual machine.