It could be a number of different things in addiion to whether you are using a MAC or PC environment. If you loaded a trial version then did not purchase it or decided to take another route, you will have to completely reomove the original trial version. Assuming this is a PC platform, you will need to get the Adobe Script removal tool from Adobe (free) and follow the instructions carefully, including a second step that involves using the Microsoft deep removal tool which will remove any traces of the former progarm so that you are sure that you are working with a clean install.
Follow the instructions for both the Adobe Script removal and the Microsoft file removal tool very carefully as there is NO return if you are not careful. I experience a similar situation and this worked perfectly!