If you need temporary or periodic access to a different Windows version or OS, you can reinstall or replace Windows, or configure the PC to do dual- or multi-booting.
However, these are permanent solutions that can be tedious and time-consuming to get working. Another option is to use virtualization.
Virtual Computer Basics
If you need only temporary or periodic access to the other OS, you can use a virtualization application, such as Microsoft's Virtual PC or Sun's VirtualBox programs.
These applications provide a simple interface within Windows (or other OSs), in which you can set up and access virtual hard drives.
Once you create a machine with a virtual hard drive in these applications, you can use the host computer's CD/DVD drive or image file to run the installation of Windows or another OS, or to run a bootable application.
The installation screens and operating system will appear within a window on the host machine, in which you can minimize and maximize it[md]exactly the way you can with other programs running on the host computer. The virtual computer and its OS will work pretty much like it would from a real PC.
These virtual computers require and use memory (RAM) just like real ones. For example, if you have 1 GB of memory on the real PC and you allocate one-half gigabyte for the virtual computer, your real PC would be limited to using that amount.
If the virtual computer requires a whole gig, you can't use virtualization and might need to install the OS directly onto the computer.
Virtual computers can also make use of most of the host's hardware. Devices plugged into the real computer's COM or printer ports are available to the virtual machines. Network adapters are also emulated.