It is the full version that I am using downloaded from the "Registered User Area".
That sounds quite weird,
There might be another case, registration button might be disabled if you already registered a previous version. In that case you don't need to enter registration key again. Simply install and use without any limitations. (only if you installed into the same folder / upgraded)
I just downloaded the file and installed, regsitration button works fine. I cannot reproduce the problem here.
Try to use ddfix tool from registered users area. Then install DVBDream to completely different folder c:\dvbdream2 for example.
If problem still happens, your serial key might have been blocked for some reason. If you cant fix the problem, the way I told above, send me your login ID or serial key in PM