Lipton, Pro Tools is the name for Digidesign's music production system containing both softwares and hardwares. Pro Tools system runs on MACs and also PCs under windows NT4. Such a system usually costs $20000, (not including the computer of course.
Well there IS some stuff costs around $700. It is not called Pro Tools, neither protool. It is calle Pro Tools LE. It comes together with a audio/midi interface called Digi001. It is a ripped down version of Pro Tools for home use, on both PC and MAC computers.
Pro Tools LE now also comes with a new sound card. This new home audio system is called Digi Toolbox XP, with the price around $500.
There is also a tryout version of Pro Tools, called Pro Tools FREE, which is completely free of charge, and independent of computer hardware and OS. It offers the ability to run 8 tracks of audio and 48 tracks of midi. It could also run RTAS plugins that do not require the TDM hardwares.
Finally just for info there is another version called Pro Tools Free 32 tracks - ZONE. It is of course also made by Digidesign:) but just modified by ZONE, and could run 32 audio tracks rather than 8.