Detecting that the user is away from the PC with .NET

Question

I have a desktop application in which I would like to know two things:

1. Is the user currently on the PC (more specifically, is he giving any input to the PC), s

Answer 1

Rather than figuring out when to run more intensive work... Consider doing your "intensive work" as early as you can, but at a lower thread priority.

I don't think your questions have an answer in pure C#, unless you poll the mouse position and observe movements... Or something like that.