Wetcloth3

That can interfere on the 2.4 Ghz band. Microwaves use a hell of a lot more power than a WiFi router and defiantly can cause interference. However, Id imagine that you might only see that if your with in a certain proximity to the Microwave. 
That is odd. Ethernet should be 100% good if all the equipment is good. Are your Ethernet cables near and power cables? Electrical cables cause EMI which can interferer with Ethernet cabling. However lower voltages like 110v that we use here in the US generally doesn't cause that kind of issue, though higher voltages can from what I have read.  Theoretically you should have no packet loss between the router and you on Ethernet. WiFi however is a funny animal at times. 

Honestly, I have seen this before back in 802.11 B/G days, with a wrt54g router. I could have the laptop right next to the router and it would be near impossible to even connect and when it did, wouldn't actually connect to anything outside. For me it was the channel, as I was living in a townhouse at the bottom floor of a huge apartment complex. Lots of wifi spots... once I started flipping through the channels I found one that worked perfectly and that was the end of that.
 
I know this is probably going to sound like a silly step, but do you have a friend that would mind if you tried setting the router up at their place to remove environment out of the equation? I just find it very hard to believe you have 2 bum wifi devices in two different systems. (technically if you already tried to replace the adapter in the desktop).