There are 2 ways of running DOS and Windows programs on your Macintosh. The first, and better, way is by using a piece of hardware called a PC Compatibility Card. This card plugs into one of the PCI slots on your Mac and uses cables to connect to the monitor and the CD-ROM drive. It is like having a PC in your Mac (just not as powerful as a normal PC). The second, more affordable, way is by using software (such as VirtualPC) to emulate an PC. It is a program that runs on your computer and from within the program you can run PC appplications. This method of running a PC application can be thought of as running a program within a program. Look below for more info, installation instructions, and a comparison.
Both programs use equal amounts of disk space (the amount of the Windows system and your PC files). The PC Card is much faster and smoother than VirtualPC, however, it cost considerably more. If you must run PC software as flawlessly as possible on your Mac, get the PC Compatibility Card; if you can live with programs being occasionally slow and want to save money, buy VirtualPC.