Is there any particular reason you wish to use file to store the trial period?
Bearing in mind that if you do use this method, a complete uninstall and re-install would re-allow the trial period.
Perhaps a reg setting is a better alternative? Also, users as standard should have sufficient permissions to read the registry (may require a admin to install 1st off).
There is a pretty good example here for using the Registry.
Working with Windows Registry using VB.NET
Hope this helps.