Hi UTM,
What Clyde said, plus: if you use a cheap ($2 here, maybe less where you are) laser pointer from a dollar store, you can get a laser diode on the cheap and use that for your light source. You'll still want to build a shield for your phototransistor, though. The laser isn't IR, but it dumps enough energy on the phototransistor to work with it. At least, mine do.
Also, because it delivers so much energy, you get a greater gap between the on and off states (when light is reaching the PT and when it's not). And distance from the source becomes much less of a factor.
If you can remove the problem of ambient/exterior light interfering with the PT, you won't need to use modulation. This makes things a lot simpler.
You can remove the laser diode from the pointer (carefully) and power it yourself--it may have a current-limiting resistor in place which you can use, as mine do, or you may need to provide one. Alternately if you don't need long run times you can just tape down the laser pointer button.
There are a few threads on this board about beam-break detectors (also covered by the term "dark alarm". If you search for them you can find a lot of stuff which might help.
Hope this helps!
Torben