I'm writing a commercial product which uses a simple registration mechanism and allows the user to use the application for a demo period before purchasing.

My application must somewhere store the registration information (if entered) and/or the date of the first launch to calculate if the user is still within the demo/trail period. While I'm pretty much finished with the registration mechanism itself, I now have to find a good way to store the registration information on the user's disk.

The most obvious idea would be to store the trial period in the preferences file, but since user tend to delete/tinker with those from time to time, it might be a good idea to keep the registration information in a separate, more hidden file.

So here's my question: What is the best place/strategy to keep and create such hidden files on Windows, Mac OS X and Linux? Here is what came to my mind so far:

Linux/Mac OS X

Most Unix-like systems are rather locked down when it comes to places a user can write files to. In most cases this is only the /tmp directory and the user's home directory. I guess the easiest here is probably to create a file with a dot-prefix to make it less visible, then give it a name that won't make it obvious that it's associated with my application.

Windows

Probably much like Linux/Mac OS X - more recent Windows versions become more restrictive when it comes to file system permissions.

Anyway, I'd like to hear your ideas and thoughs. Even better if you have already implemented something similar in the past.

Thanks!

Update

For me the places for such files is more relevant than the discussion of the question if this way for copy protection is good or bad.