Hi Kevs,
To help clarify what Joe has already said, you see THREE networks listed for your home WiFi resources because your Airport Extreme is broadcasting TWO simultaneous networks on both the 2.4 GHz band (Channel 1) and on the 5 GHz band (Channel 149). The Express has joined and is extending the 2.4 GHz network for your laptop to use, while the desktop is using the 5 GHz network.
As Joe has said, the 2.4 GHz network is really struggling against a lot of competing noise from other 2.4 GHz networks on the same and adjacent channels....which would explain why the laptop is so slow by comparison to the desktop. The desktop is linked to a clean 5 GHz WiFi channel without so much competition for the bandwidth.....and because the 5 GHz network connects at a higher rate (300 -v- 130) it can move more data than the 2.4 GHz network even if there weren't so much noise causing it to be beat down.
Using your Airport Utility to configure the Airport Extreme, you can change the name of your 5 GHz network and you can change the channel that it uses for the 2.4 GHz network......the Express should automatically follow the 2.4 GHz channel change.
I believe your 4 year old laptop should be able to join the 5 GHz network if the signal strength is high enough......I don't think the Express can extend the 5 GHz network, only the 2.4GHz network (but I may be wrong).
Hope this helps and keep working at it, I had to do virtually the same thing at my house to optimize the WiFi network(s) that my Time Capsule was broadcasting on the simultaneous 5 GHz and 2.4 GHz bands.
Keep using the Air Radar tool to see where the competition is heaviest in the 2.4 GHz band and then set your network to broadcast on the channel with the least competition.
Looking at the snap you posted I would consider trying channel 5 or 6 first, then try channel 11 and see which one works the best in your environment.
Let us know.
