Should be an easy one. What is the value of the AltGr key in the System.Windows.Forms.Keys enumerator?
I've looked through it and can't see anything obvious ('AltrGr' no, 'RAlt' no, 'GrAlt' no).
Thanks Simon
Updated - Answered:
if (ModifierKeys == ( Keys.Control | Keys.Alt ) )
{
// AltGr is held down