Well because there are many variables to this I’ll just point out the most common ones.
1) Your Ethernet card is bad. I have bought bran new Ethernet cards that had problems before.
2) Your comp is loaded with spyware. Use this to remove them.
http://security.kolla.de/ (freeware)
3) You have a Trojan or virus if you don’t already have a virus checker.
http://tinyurl.com/eod (freeware)
4) You’re being hacked. Get a firewall.