you have probably run some kind of dialer type program (not actual a virus so norton wont pick it up) the first thing to do is reset internet explorer defaults. - in ie click tools internet opions programs and click on reset web settings also you more then likley have a prog running on startup, use ms config to check what is loading on startup
click start run type in "msconfig" press enter go to the startup heading at the top
then you see a list of all the progs loading on startup you can untick the ones not to start, genarly quite obvious which ones they are but may need a hand if not sure! then click apply and restart the pc, it will tell you give you a message next time you restart the pc just tick dont show me this again. and click okay/ continue
and have another try remember you can always retick things you unticked in msconfig again if things dont start up that were there before!