There is a way to run two or more operating systems, which are installed on the computer. There are two programmes, appropriate for this purpose. The first one is called Virtual PC from Microsoft. It is possible to run another version of the Windows operating system on Windows computer with this programme. This programme requires the Windows to operate and can run only on Windows operating systems.
Virtual Box
The second program would be the Virtual Box from Oracle (Sun). At the beginning it was developed by Sun and Oracle continues to develop it.


 
This programme runs almost on all operating systems. The only requirement is that the processor must be of Intel type. So Intel or AMD is OK.
It can run on:
Windows XP
Windows vista
Windows 2000
Windows 7
Linux
Mac OS (With Intel, AMD processors)
Be OS
We can install wide range of virtual operating systems, such as Windows, Linux, Mac OS, etc. with this programme. It is recommended to have at least 2 GB of RAM in the computer. The disk space for each operating system is allocated automatically, but may be adjusted. The only one disadvantage is that virtual operating systems run slower than ones installed on the computer.
It has a memorable logo. Virtual Box is free and may be installed on almost any operating system nowadays. Also, almost any operating system can be virtualised with the Virtual Box.
If we are using Virtual Box, there is almost no difference either we are installing a fresh copy of OS to the hard disk, or installing OS virtually. Only one difference emerges when we are asked how big the allocated space of virtual OS is going to be. Usually for experiments 20 GB of disk space is more than enough.
Virtual OS enables us to run Windows programs next to different operating systems. For example, we can run a XSitePro (a web design program) on Mac OS, using virtualised Windows.
Virtual PC
Here it is how  the Virtual PC looks like:

As it was mentioned before, it is the Microsoft product and it is free of charge.
Virtual PC can’t virtualize any other OS except the Windows and it runs only on the Microsoft Windows OS. 
It can virtualize:
    * Windows 98
    * Windows NT Workstation
    * Windows 2000
    * Windows XP
    * OS/2
    * Windows Vista
    * Windows NT Server
    * Windows 2000 Server
    * Windows Server 2003
Virtual Box can be downloaded here.
Virtual PC can be downloaded here.