when you sell a computer, people are interested in the internal components, not the exterior. changing the appearance of the exterior of your computer is called a "mod" or modification. some people just like to put stickers all over the case but i don't think that qualifies. a case mod is much more than just stickers or paint, it's more like a sculpture or art. mods can be bought, but they rarely have the extravagant detail that makes something truly stand out.
generic modding tips: never take apart your computer's monitor, ever, or you will surely die. wear a static wrist strap to avoid any sort of static electricity. knowing what the heck you're doing would probably help; we're talking about custom building your own computer here after all.
your local computer store would probably be willing to help you out with this if you're a beginner. in fact, your local computer store could custom build you a machine that is at least twice as powerful as one manufactured by a big-name company, and for a much lower price. usually your local store is more interested in custom assembling of "internal" components only, however. as for custom exteriors, you could either shop online/locally or make your own. making your own isn't as simple as it sounds even if you're an established welder/sculptor or whatever; you need to keep the appropriate environment for the internal parts in mind. for example, a system needs to keep cool, therefore ventilation is needed; you want to keep things easily accessible so that parts can be replaced or upgraded, etc.
a case mod doesn't have to be complex; i've seen a computer made out of a plastic storage carton that became the focus of the party. a lot of people like windows or clear walls on their case so that they can see the hardware inside, and they'll put glowing neon wires in it. some people do prefer to be complex; i saw another mod that was a life-size sculpture of a video game character with most of the parts hidden away within her stomach. maybe you prefer sarcasm, make your windows pc (personal computer) look like a macintosh. the whole idea is to make your mod/computer look like anything you want, even a fish tank; there are no boundaries to your imagination. a lot of slideshow mods can be found online, showing projects from beginning to finish.
one of these days, if i can ever afford it, i'll have a super computer and case mod it. it will be one giant room designed to look like a space ship with little switches everywhere, and a huge monitor screen that wraps around the window view. it will also be rigged kind of like an elevator so i can make it feel like i'm flying. maybe i'll even have the room next door made up to look like the surface of mars.
…and no, a geek doesn't have anything better to do.