I'm not sure, but I believe Pro Tools 5.3 for HD systems might have been the first release that did not use the DigiSystem INIT file. If it were required for Pro Tools 5.3, and was not present, Pro Tools would not be able to launch. The file structure changed at some point, and that might have been with that release. (I'm not able to check right now) In any case, I would install Pro Tools 5.3 from your CD, then update to 5.3.1, then 5.3.1r2, both available here: