A conversion kit of that sort would allow you to install your desktop/tower computer in a standard 19" equipment rack. They usually come in one of two flavors: a kit which either has sliding rails or a shelf unit which allows you to mount the entire computer (chassis and all) in a rack, or a kit with a custom rack-mountable chassis into which you install your motherboard and system components.

Links in the following Google search provide examples:
http://www.google.com/search?q=compu...&start=10&sa=N

Usually the case has to be designed for such a task, or have a kit available to convert it, rails and all.

There are shelves like these:

http://catalog.blackbox.com/Blackbox...brm361gpcs.jpg

http://catalog.blackbox.com/Blackbox...d/rm114pcs.jpg

I found these two from www.blackbox.com. Their stuff is SUPER high-quality, but it's SUPER expensive as a result. If you can find the stuff there, you may be able to find it elsewhere, for cheaper.

Middle Atlantic is another supplier of sturdy, high-quality rack gear, but they're on the expensive side as well.