Windows: where can I store data secretly in order to implement a time-limited demo?

Question

I'm writing a Windows app that has a time limited demo. There's not going to be a server that the app can phone home to, so I need to store data on the system in order to figure out if the demo has been started and how much time is remaining. The location of this data needs to be obfuscated so that a typical user (and possibly even some power users) are unlikely to be able to find it.

I already know the logistics of how to implement a time limited demo as long as I can store data secretly somewhere on the system, but I'm not sure how to do that last part. The requirements here are:

• The data needs to be globally readable and writable so that any user account can access it and modify it without requiring elevated privileges (as the demo applies system-wide and not on a per-user basis)

• Preferably it doesn't require elevated permissions to create the data, but if it's necessary to do that once (for example to create the data and adjust its permissions so that everyone has write access) that's acceptable though not ideal.

• Whatever method or combination of methods I use to do this needs to work in Windows 7 and later

Does anyone have any idea on how I can accomplish this?