The microwave turns on when we open the door. Is there a switch that can be replaced to fix this problem

The door interlock switches require replacement.

The switch assembly is somehow staying "closed" satisfying the interlock and enabling it to run.
Disassemble the unit, beware specially of the high voltage capacitor, if you aren't comfortable or competent, buy a new microwave. If you are mechanically inclined, disassemble the unit until the switch assembly is accessible; it will have the door latch attached to it.
The switch is either broken or something? exogenous is keeping it closed. It may have come loose due to the door slamming?.
It's a simple fix. I am certainl you will be successful.

bin it there not worth fixing

please review this and fix it. God bless you

1. Interlock switchIf your microwave is turning itself on, the first thing to check is the interlock switch. This is usually the first thing to malfunction that could cause this issue. If the interlock switch is bad then it will need to be replaced.
   
2. Main control boardIf your microwave is turning itself on and the interlock switch is still working, the probable cause is the main control board. If the control board is defective it will need to be replaced.
   

The original problem, was probably caused by a bad door switch along with the fuse, now there is a good chance the control panel is damaged, you will need a professional to assess if the item is worth fixing.

You have a faulty interlock switch (which the door operates).
This needs to be renewed by a qualified microwave service technician ASAP.
A potential hazard if not fixed immediately!

Somehow you have a short somewhere or the door catch switch(s) contacts got damaged. If you are mechanically inclined, you can start by unpugging the unit and removing the case so you can inspect the door switch(s) assembly. Determine what is "normally open" and "normally closed" on the switch(s) and then Ohm them out with a multi-meter. Some of the switches are mounted on adjustment screws for propar contact on the door catches.

Sounds like a defective door switch.  DO NOT USE UNTIL FIXED!!  Or replace the unit.  Microwave ovens have door switches to prevent operation when the door is open because the microwaves are dangerous to humans outside the unit.  The door and cabinet contain the microwaves when it is operating correctly.

this is dangerous; replace or adjust your door switches the interlock switches should only allow you to turn on the microwave when the door is closed

Sounds like an intermittent door switch or a loose door switch mount. Very common.

At our site, we have critical safety information, info on door switch diagnosis and replacement, and disassembly information.

See our listing here on FixYa: http://tinyurl.com/yzjozk