microwave

Question

i just noticed that the microwave at work was not "koshered" for pesah. if it is currently clean - lacking food splashed all over the walls, 1-can i used it on pesah? 2- if it is not so clean, what would be required on pesah to allow me to use the microwave/ \ 3-would it be the same if it was an oven? thanks and i hope you are having a Hag Sameach