this is caused by the fact that both a microwave oven and most wifi routers operate at a frequency of 2.4Ghz there is a setting on some wifi routers that allow you to operate at 5Ghz, but this frequency does not travel through walls well, as far as them interfering, the microwave should not broadcast its frequency outside of the microwave, and most wifi routers broadcast in every direction, the radio waves then bounce off of the walls and envelope the room and other areas as well, it is not a line of site signal, so unless you have your computer very close to the microwave you should not get any interference at all. or of course if you stick the wifi router inside the microwave.
Hope this helps,
--Ray