Is it possible to install OS X on an assembled desktop?
1) OS X doesn't have any of that licensing/trial bs you're probably used to. There is just 1 version of OS X that you run, and that's it.
2) OS X can't just run on regular hardware without being "tricked" into thinking it's a mac
3) What's you're trying to do is against Apple's EULA, but is possible. You should see the Hackintosh project, and the OS X 86 project.
4) Running OS X on standard hardware is a very common feat, which is well documented on the internet… all you need is a simple google search query.