Firstly, disable your Anti-Virus software before heading in-game. Although the game is on the white-list, the security software will still analyze packets to and from the game which slows them down and causes lag (seen or unseen.) Secondly, don't run voice software such as TeamSpeak or Ventrillo before entering the game or while you're running the game.
See if that works, if not, we'll see if we can assist you further.