Slow internet on one computer

I recently switched from at&t to cox and hooked up everything myself. I have three computers and one of them is receiving extremely slow internet speeds. The internet is so slow that it can't even load any pages without timing out. Both the other computers have better than advertised speeds. The computer that doesn't work is in every way better than the other two. The computer that doesn't work with cox works perfectly fine when connected to at&t. I've been through hours of tech support on the phone with no help.