If it's a new computer remember they usually come bundled with trial versions of either Norton or McAfee security software, so check the setting to ensure the browsers are not being blocked.
I have got a really strange internet problem in vista home premium. Windows says that it is conected to the internet, cmd->ping google.com replys perfectly but if I go to internet explorer of Firefox it says unable to conect to google.com or whatever website I visit... Please help!!