Actually its pretty simple to install a PSU, just make a note of which wire goes where, take some pictures with your camera or mobile camera, then unplug wires one by one. And just do the opposite to install.
Ok, it seems to be getting complicated, this page will help you : http://www.hardwaresecrets.com/article/362
Here is a video : http://www.youtube.com/watch?v=xvClS_M-09o
And I won't do it for free also, you owe me a "Pizza". How about that!!!!!! Good luck and don't get nervous you can do it.