I just write my first C# application, which is a scheduler. Once an hour I want to pop-up a dialog and lock the screen for two minutes in order to take a break.
As of now, my application just shows a form on "TopMost" when its time to break and hides it two minutes later.
How can I lock the screen as well? Something similar to the UAC style in Vista.
Eep, misread your question. The UAC dialog of Vista creates a new desktop and shows itself on that. To Create a desktop you can use the CreateDesktop function. You can then use SwitchDesktop to switch to it.
If you truly want to re-create the secure desktop's looks, then you need to take a screenshot first and display that on your form, darkened a little.
Original answer below, which might be of interest too:
Take a look at the LockWorkStation function.
However, you can't programmatically unlock the work station again. That's something the user has to do himself. So you can't easily enfore a break at least two minutes long unless you still resort to your top-level window technique to deny the user his workspace as long as the two minutes are not yet over.
As an alternative to taking a screen grab (which I've also used as an approach in the past under slightly different circumstances) you can also do this by creating a new window (WinForm) that is full screen and on top of all other windows.
If you set the window background colour to solid black then set the opacity to 70-80%, you'll get something that looks like the UAC prompt in Vista.
e.g.
formName.WindowState = FormWindowState.Maximized;
formName.FormBorderStyle = FormBorderStyle.None;
formName.Opacity = 0.70;
formName.TopMost = true;
Of course it would be sensible to draw a window on top of this informing the user why the screen has been dimmed, with a counter.
As with the screen grab approach, this on it's own does not prevent an user from by passing it by using the Windows key, Alt-Tab or Ctrl-Esc to bring up the Start menu, or to switch to other tasks. That's a bit more tricky, but there is a good write up for how to do that in C# here:
http://tamaspiros.co.uk/2007/12/20/c-disable-ctrl-alt-del-alt-tab-alt-f4-start-menu-and-so-on/