Open Question: Download speed too low! Please help!?

Hello everyone,
So, basically, about a week ago, my dad bought a brand new ASUS router, ASUS modem and a very fast 10mb/s UPC high speed internet connection, Before, at home, we has some sort of old router/modem and a slow internet providor, and our internet was very slow(download about 150kb/s).
I was really happy, because I will finally be able to download at a really high speed, Play online games smoothly and watch movies online in HD.
The first two weeks, everything worked fine, our download was about 450 kb/s.
But suddenly, A couple days later, Our download speed rapidly decreased. Although our connection speed was still good, our download speed was only about 20kb/s. Which meant downloading a movie or a song took ages. I thought this has something to do with UPC decreasing the download speed on illegal websites, But even official microsoft or other websites has a really slow download speed. I am not downloading from torrents or anything, and I know whats the difference between kilobits and kilobites. I just need some useful answers!

I will be grateful for every answer! :
Thanks :)