how to disable "PRINT SCREEN" button while running my Application in WPF?

Okay, it is possible, and could indeed be useful if your application is deployed in an environment where a camera is not available to the user.

First of all, I used the RegisterHotKey and UnregisterHotKey API calls, documented here http://pinvoke.net/default.aspx/user32.RegisterHotKey as described in this rather old article here http://msdn.microsoft.com/en-us/magazine/cc163713.aspx.

I registered the IDHOT_SNAPDESKTOP hotkey in the Window_Load event and unregistered it in the Window_Closed. Trying to do this in the constructor gave me problems getting a consistent handle with the WindowInteropHelper(this) method.

If you'd like to do more than just ignore the keys you can set up a windows message handler, making a kind of WndProc using,

HwndSource source = HwndSource.FromHwnd(<handle>);
source.AddHook(<WndProc>);

making the handle as described above, and the WndProc implementation yourself.

As yet, I don't know how to "not" handle the hot key and get windows to perform its normal behaviour except, of course, by unregistering the hotkeys.

Its not very elegant or "WPF" but it worked for me.

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As @ghord comments

The use of EnsureHandle() looks useful for getting a handler in the constructor.

It's not possible to disable printing, and even if it were possible, it would be easily circumvented by a cell phone camera. Many are in the megapixel resolution range, making it quite easy for someone to get the information they want.

If you want to disable the Print Screen Key on your keyboard, Jodrell's answer gives a way of doing that (understanding that it's not going to keep people from printing, and a determined user will find a way around that).

Really, it all comes down to trust. If an employer can't trust their employees not to remove data that is already protected by law in most jurisdictions (HIPAA in the USA), then there's a bigger issue at stake.