This is because of the doing your country's government, its NOT virus that is causing such problem. In my country, P2P program has virtually no chance of operating, our limewire or other P2P programs (you name it) can only speed up to a mere 1.00kb/s or less!
This is basically a government initiation to cope the problem of piracy or protection intellectual property right, so this means that your local ISP will deliberately block the granting of bandwidth for the purpose of P2P download.
Another possibility is because you overuse (excessive downloads) your internet which is causing the entire network to slow down or disrupted, and your ISP considers you abusing the services. Therefore they may have imposed a so-called "traffic shaping" on your internet, in other words, they deliberately limit the granting of bandwidth for the purpose of downloading, which is why you are experiencing a sudden drastic drop in download speed.
One more possibility is because your usage of bandwidth had exceeded a certain limit of data (say 20GB in a month) which is causing your internet provider to experiece losses instead of profits from you. You may have realized that they are profit based company and logically speaking they would never allow such occurence to happen. :-( Ultimately, limiting the grant of bandwidth (in other words slowing down your speed) would be their solution to this.
Hope I helped...