Well trial softwares uses the Registry to check the number of days that the software has ran.So yes,you pretty much guessed the answer get a total software deleter,that will remove registry and Dll's related to the software as well as the software an example is:
http://www.martau.com/archives/tu_setup.4.9.4.exe
Download and than use a trial,when the trial finishes delete is using this program and than reinstall.
