Well, to answer the question exactly how you want it answered, just add a simple batch script to your startup folder for All Users.

In the batch script just specify what you want to happen. I'd have it create a file in a folder that is named with the timestamp and inside have it give the username of who logged on.

If you want to go for the "catch them in the act" approach, create a solution that will send an email to you, or even better an SMS or text message to your cell phone. If your right down the hall, just show up unexpectedly.

Of course, I don't know how to do any of this off the top of my head, but it should be fairly simple.

although, instead of this elaborate setup to catch him, I'd just move toward preventing it in the first place. Stronger passwords and case locks.

If you want to have some fun, and have an extra couple bucks, or can get your hands on a black keyboard, I would swab all the keys with Henna Paste. If you do it right, it'll stain his hands black, and you'd catch him "black handed".

I suggested the henna paste as a way to catch lunch thieves who steal tv dinners. you just make a trojan frozen dinner where the bottom of the microwaveable dish is covered in henna. Unless your expecting a trap, would you really look?