I had this with mine for a long period last year. Finally realised it would start doing it after, if I recall, a indows update. Finally ditched Norton whichj I was using and started using another virus protection programme (McAfee) when I went from dial up to broadband.
It was only when I was installing it and reading the instructions that I realised that I had been using both the Norton firewall & the Windows firewall at the same time.
So I think there was a conflict between the two firewalls.