Steve have you checked for spy ware, cause that will rob you of a lot of bandwidth, one program isn't usually enough. I would suggest the following 3 they work well together.
http://lavasoft.element5.com/software/adaware/
http://www.safer-networking.org/index.php?lang=en&page=download
http://www.javacoolsoftware.com/spywareblaster.html