Do you scan for spyware?

I have run ad-aware.

 

Run and second source and compare results.

Here is a link to the speed test site.  You can test your link's speed.  http://www.speedtest.net/

 

Are you connecting directly to the cable modem with an Ethernet cable or are you connecting to the modem with USB? (Ethernet is preferred).  If you have a router between the PC and the cable modem, bypass the router and connect direct to the modem and see if that makes any difference.

 

Finally, have you had Comcast check the link? 

Is it all websites or just those listed. If it's all of them I'd check with your ISP first to have them rule out the service. Try a ping test to you gateway and beyond. The speed test won't be any help because Comcast just added the Speedboost and it unfortunately gives false reading on speedtests.