Internet super slow, for no apparent reason

For the past 4 weeks, my internet has been stupidly slow, and with slow I mean it feels like I'm browsing with dial-up.
I first noticed this in my computer, but when I told my roommates, they tought it was just my computer, because everything was fine with theirs. Two weeks later, they also started noticing. But the strange thing is: it is slower on my computer. At first I thought it was because I was browsing from the basement(house foundations and isolations could be and are interfering with my wifi signal), but today I took the pc upstairs, and I was right next to the router and google took like 30 seconds to open. I did the same on three other computers and it only took 5 seconds, still longervthan I would expect, but its way faster than mine. I've changed browsers and everything. I believe my drivers are updated, I've tried shutting down my firewall and antivirus, and that also didn't speed up the internet.

Other info: Steam registed a download speed of 700kb/s, which is way too fast. Opera registed a download speed of 500kb/s. The max I could get out of chrome was about 75kb/s. Sometimes it doesn't even connect to te internet, it detects the router, but isn't able to connect, for some stupid reason. I have cleaned up and defragged everything I can think of.
I'm using an ADSL wifi connection with a speed of 15mbits.
Netspeed registers 3, sometimes 6mbits/s, which are values you would expect if you're using wifi. Other computers register 8-9mbits/s.

Please help!

Additional note: I'll be changing my internet provider in a few days, which will guarantee an internet speed of 24mbits/s. still ADSL because were I live there isn't optic fiber. But I don't think it will really matter, since it seems to be a problem with my computer.

Have you checked for any malware on your computer?

I've considered changing my modem, but the thing is: This modem is connected to a tv box (idk if you have that anywhere else in the world) and I cant change it because of that