I've written a small utility that I wish to sell for less than $10.

My primary concern is "casual piracy". The scenario that plays out in my mind is this:

User buys the program, enjoys using it and tells their friends. The friends copy the application to their USB drives and take it home - using the application for free (maybe never realizing they should have purchased it.

Since I've got absolutely no protection built in, it would just be a simple copy'n paste to pirate the app. The users who would be using the app are in close proximity to each other (work in the same environment), so casual piracy would likely occur frequently.

Any ideas? Keeping in mind the app is cheap (partly to reduce casual piracy), and the level of effort to write the app hasn't been very demanding.

Update: the app will run on any system that supports .NET 3.5.