Very odd internet situation

So I build a pc for someone (cheap asrock mobo with a fx-6300) and I installed windows 8 and some apps etc. on my lan, it worked fine.
But when he took it home the pc said it was connected to his network, but Assasins Creed takes 93 hours to download, it could't load websites or it would take 30 seconds+.
So I assumed it was his connection that was that bad, but on the multiple other computers they had it would have 600kb/s and would load websites fine and such (he also has a usb wireless thingie which has the same problem, it works fine on other computers, and not on his).
I tried the obvious things and it didn't work.
So I hope you guys have a solution.
Thanks in advance.

That's pretty freaking slow, if it's Kb/s. Not so much but still slow if it's KB/s...

Hint: upper case B is for bytes. Lower case is for bits. There's a significant difference, literally a รท8 or x8 difference.

Erhm, right, if you could list these "obvious things" that'd be fantastic.

If those download speeds are in Kb/s (Kilobits/second), that download time for assasin's creed seems about right. It's a 40GBish game, downloading it in around 100 hours, according to my math, would mean it's downloading at about 0.9Mb/s. Just a touch more than that reported 600Kb/s.

With 600kb/s I meant 0.6MB/s (megabyte) obvious things were turning off and on virus scanner and using different cables in different ports (on the modem).
But the pc next to him, using the same modem loads websites fine and such.
This pc won't load webpages, or it takes really slow (30 seconds)

So, what, every computer on the network was at that speed?
If no, what speeds were the others at, and what's the specs of his system?

I can't remember the motherboard, fx-6300 8gb of some corsair ram, radeon 380 msi gaming edition corsair cx-600M.
He is now running speedtests, I will come back to that

On his newly build pc his ping is 91 ms, down is 0,90 Mbps, upload is 0,03Mbps.
On a other pc with the same exact cable as the pc we just tested it is: 23 ms, 6,0 Mbps, 0,64 Mbps,
His mobile phone tested with wireless internet is: 25 ms ping, 6,12 Mbps down, 1,26Mbps up

Using the latest drivers etc.? I'm stumped.

I downloaded all the drivers listed on the asrock website for this mobo.
I think it was realtek xlan for the internet.
Also they let a ''proffesional'' look at it, he said the ''software'' on the pc was optimized for ziggo (my isp) he uses KPN as his isp. He also said nothing could be done.

Well, he's got an AMD GPU too, this might be relevant:

OMG... i just solved this on my own.
I was looking under my installed programs since this was a fresh install and i noticed something that i had never seen before called AMD Quick Stream which i assume came from my AMD all in one Driver. I went to programs and opened it up and turns out it seems like this program regulates internet flow to programs based on demand and need. You can have it allocate bandwidth and even reserve bandwidt for certain programs and it increases these bandwidths based on how much its using..
this explains everything.. it explains why ONLY my computer was getting crappy tests but everything elses was fine. it also explains why my speed tests slowly got faster and faster every time i tested because it was allowing chrome to use more internet every time i tested the speed because it was hitting the current limit. It also explains why the speed would slowly go up EVEN WHILE the test was happening.
i uninstalled this... reset the computer.. and BOOM... 34Mbps

I would like to add that it is not a legit bought copy of Windows, but I do not think that could influence this kind of stuff.

I asked him to use the windows 8 search and type in AMD Quick ... but nothing came up