the Best way is to run an ethernet cable to a second location, and place another wifi base station there (an airport express would be an OK choice), give it the same SSID and security settings, but a different channel. and turn off the "router functions" (like DHCP) of the router (if you go with the express, you want to put it in bridge mode on the internet settings page)
this will make your network a bit more stable, as the connection back to your main base is wired, making it more robust. Also wirelessly extending will drop your speeds if you are connected to the remote base. The speed drop will probably not be noticeable on your internet connection, but for transferring files from one computer to another on your network, it will be.
you could also an airport express to wirelessly extend, just put it somewhere between the main router, and where you want to extend to. When doing this i recommend running an ethernet cable from the express to your computer to set it up, it will make the process much smoother, and then you can unplug once it's working.
wifi = 2.4 GHz, microwave oven =2.4 GHz, so it sounds like you have a poorly shielded microwave, it would be like you trying to talk to someone on the other side of the room, while someone stood in the middle and screamed at the top of their lungs.