FYI: Because DVDFab makes use of the VSO DVD burning engine (the Patin-Couffin CD/DVD access sublayer in Windows) in order to successfully register your copy of DVDFab it must be installed FIRST.
I discovered this when I installed CopyToDVD 4 (which I am also a registered user of) on a clean installation of Windows XP SP2 with all of the updates applied first and then installed DVDFab and was not able to activate it with my key.
After removing both and restarting the PC, I installed DVDFab first and it registered successfully. I was then able to install CopyToDVD 4 again and since VSO has their registration key in the Windows Registry, both programs now work successfully.
This may have something to do with the fact that both programs try to install
the CD Access layer.