uninstall Norton, take the cd and hit it with a big hammer.....nearly all the computers i deal with with this kind of problem ALL have norton installed.
joking about the hammer bit. lol
seriously though, use the free AVG click here and the free Zone Alarm click here for the security, if you have a router running between the internet modem and all the computers then you may need to check the router settings, read the manual to get to the settings page (Belkin use 192.168.2.1) type it into the address bar of IE or whatever browser you use. and check the settings
one last thing, do you need to use Incredimail?, if not then dont, that might be the cause of the problem as i know it has on a few pc's i have repaired (but not all).