Just an FYI based on personal experience. Try the reflector based TrackClip that came with your TrackIR, before you spend $40 on the TrackClip Pro with the IR LEDs. The only reason you should need the Pro is if your PC setup is in a room that is brightly lit (sunlight). The TrackClip that comes with the TrackIR actually works better than the Pro, especially if you are in a darker room, like a basement, etc, since the TrackIR unit is providing IR illumination from the unit for the TrackClip. Just thought I'd share, since I built a homemade IR LED array and had mixed results. After watching a few youtube videos, I grabbed my TrackClip and mounted it to my headset, and WOW, functions great! Anyways, hope I can save someone from spending $$$ or time unnecessarily.
Ok, a little more experience here. The TrackClip setup definitely does not like sunlight and/or glare directly behind you. Tried opening the blinds in my basement to make it less cave-like. Fired up my TrackIR and it wigged. Didn't calm down until I closed the blinds behind me. Off to the side seems ok. Once I did that, everything went back to normal.
or just buy the track clip $40...skip the TrackIR and download the freetrack software and use it with a webcam.
Just got mine set up today with the method sam suggests, works a treat! Had to make a lense for my webcam to filter out some of the visible light, but that only took 5 minutes and a piece of developed negative film. Only issue I had was that when looking right it would zoom in ingame as the clip moves slightly closer to the webcam, I probably just need to mess with the curves or something but for now i've just set that axis to a zero value. Any suggestions?