What have you all used for "expiration" logic

[szlamany]

I've got an app that we want to allow users to have for a given amount of time and then have the app simply stop working.

I was wondering what any of you might have used at a commercial level to achieve something like this.

I don't mind sending a new .EXE every renewal "season" but it would be easier to send a new license key.

btw - we connect to a SQL server with our app.

I noticed that Sony Vegas software takes a CD key and then after install the app gives up a "computer number" - based on specifics in the hardware/OS installed on that box. You give them these two figures on the website and they give you an activation key.

That technique sounds nice - since we have a central SQL box - we could gather a "computer number" off of that...

Any help would be greatly appreciated.