Being extremely paranoid, I needed a nick that would adequately disguise my true identity. So I came up with the rather cunning plan of prefixing my real name with the letters "Mc" :)
And the real story is that Darren is a pretty boring and common name, so I'm forever running into nick collisons whenever I try to choose it as a nick|handle. So I usually fall back on McDarren or MrDarren or something similar.
The odd thing is that Darren is not (yet) taken on PM. I have no idea why I didn't use it - perhaps I just assumed that it would have already been taken. *shrug*