We do this in one of our applications. We start the trial in the program from a certain point in program installation. The basics are as follows:

1. Make a registry entry containing the start date of the trial. We encrypt this though, so if a tech savy user goes into the registry they don't see a plain date to change.

2. Have your program check this registry entry on startup and various other key points in the program. If its 30 days after this disable the program, or parts of the program (its up to you).

Now, there are a few issues with this system. If the user changes their system date then they could get around this. However, must people won't change their computer date as this affects other programs and your program.

This is a very basic system, and you can get even more savy as time goes on, but its a nice start.

Hope this helps,
Bryan