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=computer+%22rack-mount%22+%22conversion+kit%22&hl=en&lr=&ie=UTF-8&start=10&sa=N
DMR
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alc6379
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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.
DMR
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