I recently started renting out a room to someone who has a Windows-laptop.
Everything we have is Apple (2 Macbooks, 1 iPad, 2 iPhones, 1 iMac) connecting to an Apple Airport Extreme which extends the internet from the modem/router we got from the ISP. The network is perfectly stable and I have no problems using it in the other apartments in this building. (3 apartments in total).
But since the new renter moved in, the Internet gets really unstable as soon as his computer connects to it. It's not the WiFi-signal, because that works fine (I can stream from one device to the other without a problem when the internet is "down"), but when he connects his computer, the Internet just stops working about every 10 minutes. It takes a minute before the internet is back up again.
One weird thing: Torrents keep on downloading and uploading, even though the Internet is "down" (I can't browse to any page while it's "down").
I tried Google, but the results all point me to Windows laptops crashing when they connect to the WiFi, not the Internet itself. I checked and it's on every device using my Internet connection. Also my Wii U and my GF's Kindle (I have nothing connected through an Ethernet-cable, so I can't say if that works). Also, whenever classmates come over to do homework, they can connect without a problem and without killing the Internet connection.
I hope someone can help me out, because I can really not live with an unstable Internet-connection for 6 months, haha! Now using my iPhone for tethering, but I would much rather just use the WiFi.