My internet has slowed massively in the past 2 hours?

[tumbleweed_biff]

A number of possibilities:
First: malware infection
Second: peer to peer software - torrent software - do you have any of this running?
Third: number of computers on line ... are there multiple computers using your connection? Do you have WiFi running and if so, do you have it encrypted with WPA2 or is it open and therefore accessible to anyone nearby?
Fourth your internet connection: if you have cable modem service then you share bandwidth with your neighbors.
Fifth, Some ISPs offer a burst service to make it so that smaller downloads occur fast but then longer downloads are at a more sedate pace.
Sixth, your download speeds are limited by the outgoing bandwidth of the host.