Greetings korniceman3000 and xXAlphaXx,
I hope I can provide some additional insight here regarding the options available
to install both XP and Vista or OS X. If I may first address xXAlphaXx 's scenario,
commonly known as dual booting, is quite involved and cumbersome for the first
timer to this approach. All-be-it, you're here to learn about the process, so in that
manner, let's get into the options.
Dual Booting is the process of booting (starting) one OS or another from the same
piece of hardware (hard drive), by using something called a bootloader (used to
get things going by the boot straps). In order to do this you may have to mess around
with the Bios and/or the Bootloader, which can be problematic.
Another option, Virtualization, is often much easier, especially in the context we're referring
to. Virtualization, allows you to run one Operating System normally (A host) and a second
Operating System (Guest) inside the Host as an application unto itself. The benefit here is
less work, and more productivity. What's even better, you can download the software for this
for free from Microsoft, or several other.
Virtual PC, VirtualBOX, and Parallels (Mac OS X) are all examples of virtual machines (The software
that allows you to run a Guest operating system). All you need to do is choose a Virtual Machine
software that fits your needs and compatible with the Host operating system.
The Tech Saavy user would even go as far as getting a pc with Hardware Virtualization (The ability
to maintain performance in both operating systems). For instance my Dell 1720 with an intel T8300
and hardware virtualization runs Vista and XP perfectly. My old applications run exactly as they should.
Finally, if you run Vista, your XP applications can use something called compatibility mode (Run your
old applications as if XP was installed without a Virtual Machine) Using Compatibility mode works
great for many applications written for XP and is by far the easiest solution (Some Applications simply
won't run in this manner, but the number is quite low.)
If you have questions about how these processes work, don't hesitate to ask us, "google" it, or check it
out on wikipedia.org
Hope this is helpful.