There are many ways of doing this on both a low and high budget. Most of them boil down to the use of a micro-controller to handle the sensors and then interface it with a computer (either with proprietary software or something you write yourself). The BCS (posted above) seems to be a very easy to use system and they have done most of the hard work for you.
A common "low budget" system might use an Arduino micro-controller (www.arduino.cc
) which is very much a DIY sort of project.
If you are comfortable with electronics and programming then this is usually fairly straight-forward.