Internet connectrion grinds to a hault while specific computer is connected

The internet connection we have here isnt great to begin with. But when one of two specific computers is connected to the internet, our connection drops to almost nothing. This doesnt happen all the time, only some of the time. Disabling their connection fixes the issue. Specifically the upload speed doesnt even register according to speedtest.net, and the ping usually reaches 1500+ms. I have ran virus scans with malware bytes and a few other programs, but nothing showed up.

This started happening about a motnh appart between the two computers. But the owners of them visit similar websites. Whats the next step?

via Bleeping Computer Last 20 Posts http://www.bleepingcomputer.com/forums/t/511361/internet-connectrion-grinds-to-a-hault-while-specific-computer-is-connected/