Why would you ever want to run multiple operating systems on one computer? A computer / network technician may need access to many OSes, possibly all at once. But what would a regular user want with more then one OS? There are many reasons why someone may want to do this, the most common of which is that the user prefers Linux or Unix (Mac OS X) for the speed, security, and stability but occasionally need to run Windows specific programs. Now, you can run some apps under virtualization without Windows by using utilities such as Wine or CrossOver. However, it may be easier (if you have a Windows disk around) to install Windows along side your favorite OS. There are two methods for running multiple OSes; dual booting and virtual machine. One method is not better then the other but we will discuss the pros and cons and find out which method suites you best.
Dual booting offers the most power. With the dual booting method you install the second OS on a separate partition and, during the computer startup, choose the OS you want to use. Usually the feature allowing you to choose the OS to boot into is automatically setup. If, for example, you install Ubuntu on a PC that has windows on it it will install the GRUB boot loader automatically. Also if you install Windows on a Mac then you can hold the alt/option key during the startup chime and choose the disk to boot from there. Once booted the OS will run at full power because it will not need to share system resources with another OS. This is ideal for high-end gaming, graphics, video, 3D rendering, etc.
Running a virtual machine is more convenient but lacks the power because it is sharing RAM, Video, Processor, etc with your primary OS. There are many VM applications available including VMware ($ Mac & Windows), Parallels ($ Mac & Windows), and VirtualBox (Free, Linux, OS X, Windows). With these programs you can install and run another OS without ever shutting down the PC or leaving you primary OS. You can use a virtual machine as you would any other system baring some high-end heavy graphics stuff. So if you absolutely need a single Windows program like Outlook or you just want to play with good ol’ Windows 3.1 then consider a virtual machine. Also VMware Fusion and Parallels have the ability to run your Mac’s BootCamp Partition as a virtual machine so you can have the best of both worlds.
What is the best way? I don’t know, you choose. The process for setting up VMs and/or dual boots varies drastically depending on your hardware configuration and the software’s you are using. There are tutorials online but just backup everything and give it a try. Things may go wrong the first time but keep trying. Once you get it working you just might be glad you did it.