I guess the first question would be “Why should I root my Desire?”. The main reason is it gives you more, if not total, control over your phone. You get access to parts of the hardware the standard OS wouldn’t risk letting you go anywhere near. Two useful examples being the ‘Use your LED Flash as a torch’ app and an app that lets you control the CPU speed. Both of these theoretically let you damage the hardware of the phone so if you do decide to root, you should be real careful what you then do with it.
Okay, with that warning out of the way, lets move on to how you actually root your beloved Desire.
I should stress at this point, I didn’t work any of this out, I didn’t program/hack anything, I just thought it best to put all these bits of information into one place, instead of three or four.