Parallels Workstation is a commercial desktop virtualization program that lets you run multiple Operating systems on the single physical computer and you can easily switch between them without rebooting.
For instance, you could be running Windows Vista and but some of your programs (like your old Tally accounting software) require Windows 2000 or XP. In that case you can download Parallels Workstation, create a virtual machine for Windows XP and run it over Windows Vista just like any another software program. You can also use Parallels Workstation to run Linux on a Windows machine or vice versa.
If you are planning to rebuild a computer, it is recommended that you use a virtualization program like Parallels Workstation for all experimentation work – i.e., any software that you plan to use on a temporary or trial basis should be installed on the virtual machine and not on the main computer.
Parallels Workstation sells for $50 but you can request a licensed activation key for free using this Lunarpages offer. Key in your email address and they’ll immediately send you the download links for the trial version and serial number to convert the trial into a full retail version.
Virtual machines are very helpful for web designers as well as it lets you test websites on different browsers – you can have a virtual machine with IE 8 and another one with IE 6 while your main desktop is running the latest IE 7.