I'm not even sure where you can get DOS anymore - probably at a "mom and
pop" computer store in your area.
If you want to run DOS as an OS on your computer, you'll have to wipe your
hard drive and install it, then install XP again in a dual-boot environment
(there are probably other programs - like System Commander? - that would let
you do this without reinstalling, but I don't know enough about them to
advise you on their functionality).
Your other option - and the one that I'd recommend - is to purchase some
virtualization software, like Microsoft's VirtualPC or VMWare Workstation.
This will allow you to have a virtual computer inside of your current
operating system without actually changing any of your system's settings in
real life. You can install any PC-based operating system you want in the
virtual machine without having to worry that it might screw up your
computer. It's a really nice option to have, if you ask me.
I haven't checked prices lately, but I believe that both of them cost around
$200 USD, one may be more, one may be less. However, it's definitely a nice
utility to have if you plan on playing around with other operating systems,
but don't want to risk damaging your current OS.
Good luck!
--
Mike Kolitz MCSE 2000
MS-MVP - Windows Setup and Deployment
Remember to check Windows Update often,
and apply the patches marked as Critical!
http://windowsupdate.microsoft.com
Protect your PC!
http://www.microsoft.com/security/protect