Well you should scan your PC for Malwares,Trojans,Viruses,Spywares..
Your PC would be infected with some of them that's way your internet is going real slow.
Another possible cause is that would be something wrong with your cables,Also maybe something is sucking your internet speed like a program or something or maybe something wrong with your router,modem,Lan card or whatever ETC.
Check those things first and i hope you find the cause...
Good luck pal!