Wikipedia says a microwave oven consists of:
a high voltage transformer, which passes energy to the magnetron
a cavity magnetron, which converts high-voltage electric energy to microwave radiation
a magnetron control circuit (usually with a microcontroller)
a waveguide and a cooking chamber
It is difficult to say what's wrong with it without hands-on diagnosis and it's likely that you do not have the equipment to do this safely. You probably need a specialist to repair it - that or a new microwave.
Good luck...
Peccavi
March 2009