Have you tried to power cycle all your network equipment?
Any one else on your network downloading stuff?
Have you tried multiple web sights, some sight will limit your download speed to there servers so there internet doesn't get hammered.
Try downloading with different web browsers, and if you have another computer try that as well.
If that doesn't work call up your ISP and ask them to do a test, they might have a failing switch or something in there line
For my speed test I like to download a torrent of Ubuntu, that has alot of peers to suck off from, that usually max out my internet speed. Steam is also a good one to do if you have an account download a demo or a free game.