Trtial software key creator is a freeware that works well
http://www.softplatz.com/Soft/Development/Management-Distribution/SoftwareKey-Trial-Creator.html

>Hi All:
>
>I want to add a time element to the eval version of my product. I need something that persists even after the user uninstalls it so that if they do re-install, they still get the dreaded "30 day trial over. Do you wish to purchase?"
>
>Thanks.
>Luke