I think it leaves something to the registry or somewhere, just to preven ppl like u using trials all the time.
Though I have heard that one effective way is to turn your computer clock so that u like make it year 2007. Then install the trial, and then turn the clock back. Though I don't know if this will work and u would need to turn the clock backwards since u have already installed the trial. So try turning the clock back to when u still had time left on the trial
Though I have no idea if this will work, someone said it works sometimes, but I doubt it works often.