deltatz

Guys i need help here,my project is design a time table program for schools.the idea behind is that the program should sound an alarm each time class periods ends which i dont have a problem up to that point.im fluent with Visual Basic.My problem is i want to make the program run on the machine while the machine is running other program such as msword,excel etc.in other words when the time for the alarm to sound comes there should be a machanism to interapt the current running program,serve the alarm and return back to the interapted program without the user noticing anything.that is to say it should run stealthly.Im desparate for the tips guys.Please help,thanks in advance