Microsoft has made Virtual PC 2004 for Windows available for free download. Note that it is only the Windows version, and not the Mac version. Virtual PC is a virtualization suite, which allows user to run multiple instances of operating system at the same time.

Microsoft only officially supports some versions of Windows and OS/2 to run on Virtual PC. However, since Virtual PC emulates a Pentium II processor and other common PC hardware, many operating systems that runs on such platform also can be ran on Virtual PC. This link provides a comprehensive list of operating systems and their compatibility with Virtual PC. Most of the popular favors of Linux and *BSD can be used with Virtual PC.

Guest operating systems that installed with Virtual PC are not meant for day-to-day use. However, it provides a quick and easy way to develop and test on multiple platforms.