To capture special key clicks (like ctrl, alt and shift modifier key clicks) one does not have to use KeyDown & KeyUp method. There is a class Keyboard which is available in Silverlight as well. Use Keyboard.ModifierKeys to accomplish desired effect.
Keyboard.ModifierKeys allows a bitwise combination of its members.
To capture other key clicks one needs to write a static class which will be available in the whole application.
The example KeyPressMonitor class is available here.