2 PCs, 2 very different DL speeds. HELP

You're problems are not isp side.

You have some faulty hardware somewhere.

OR

You have traffic controls in your router that are messing with you.

If you wanna test to see if your cables/router and able to move the data, just move a large file from computer to computer using filezilla.

If that checks out then there's probably some rule in the router's traffic controls throttling you for outbound packets.

I would suggest you also pick up a cable tester. Cheap ones cost around ~35$ and will do the job to tell you if an UTP is messed up or not.

As for the cables that may be in question, you have gigabit ports but only have 100base T cables, so you are throttling yourself there anyway. But its good up to a length of 100 meters.

The ISP called me back after I bitched on twitter, they told me there is actually some packet loss problem.
The technician came to my house.
He concurred that the problem is at the place in the neighborhood where are located all their control units (I dont know how they're called in english).
Tomorrow he'll go there and hopefully solve this shite.

If you have another in the network getting 60+, I don't think a tech would do much. Not sure where you are, but ISPs here will say things like 'this coax splitter is not comcast approved, let me give a shit one'
Did you check your net adapter settings? I'd suggest a reset, you can use 'netadapter repair' which will run the cmd stuff for you.

earlier I wrote that when I tested the cables I re-tested the other pc and it got shit connection as well.
Doesn't matter, it's solved now

I have the same issue on my LAN. My bro and my PC are both 3930Ks, same NICs and everything but he pulls in 32-34MB and I pull in 27-30MB, sometimes it's the drivers sometimes a PC will take more traffic. His steam will DL @ a full 3.0-3.8MB/s and mine at most 2.8/3.4MB/s. His ethernet cable is worst off and it has cuts and dings on it and longer and I'm closer to the router and my cable is better off. Sometimes you might wanna check the router system log. I've been noticing my brother's PC does alot of DHCP requests and mine doesn't even do any on the router's system log.

Ya might wanna remove some metal and radio interference near your PC such as bluetooth and wireless, it might help as well.