Is it DSL or cable broadband? It may be ingress in the line. Meaning over time water soaks in the ground and affects the drop to the peg outside. If your other devices behave the same way it could be that, or your modem or router. If it's just one device than obviously it's Windows or your hardware.
There's also no way you have 0 for an upload speed. In order to use the Internet you need some upload speed. So whatever website your using to test Internet speed is giving you bad info.
Try speed.cloudflare.com and fast.com. Now add both results together and divide by 2. That's your average speed. Also, at Cloudflare's website, do you see a very high jitter?
Another way to test your drop to the peg outside is to use broadband report's online smokeping. You'd have to allow ICMP pings for 24 hours in the router or computer and after 24 hours you can see how your line behaves as it's tested from a server from the outside. If there's a lot of high ping or no ping you more than likely have an issue with the router, modem or drop.
I have found both of those Internet speed testing websites to be accurate. At least for me anyway. It could vary for others due to their location to the server those websites use and the hops it takes to go to that server and other things. Online speed tests are more or less just a round about way to test your speed. It's never something accurate. Besides, just connecting to different websites and doing different downloads will all vary due to their location, server capability, speed limits imposed and lots of stuff. That's why when I got fiber I chose 350 Mbps (43.75 MB/s) because that download and upload speed is more than enough for me, it's plenty fast, and I don't need some massive 1 Gbps since most websites and whatnot won't even dish that kind of speed out in the first place. As it is now with my 350 Mbps using Fiber there are many websites that limit download rates to a piss ant amount of some 2 MB/s or maybe ~6 MB/s. I could pull 43.75 MB/s. Some websites do dish out the speed and I notice it when I can download a ~100 MB file in about 2 to 3 seconds. Since this is fiber my upload is the same rate which is great for the cloud backups I do. I've actually backed up whole Windows operating system ISOs to one of my cloud providers in about 3 minutes or so, and it could have been faster if they didn't impose an upload speed restriction. 1 Gbps is only good if you have say a family of five all streaming Hi-Def content and playing video games with Steam or whatever. Then you'd want that 1 Gbps speed simply for the needed bandwidth.
Something else I'd look into. The quality of your Ethernet cable if you're using that. If you're using Wi-Fi it can and will be a nightmare if not setup properly and loads of other things. To get a true online speed test you'll want to use Ethernet cable. I've wired the entire house with Ethernet and use mostly that, outside of the devices that have to use Wi-Fi like phones, tablets and whatnot.
