this sounds like a home page hijack on ur core network computer/server which the council IT dept ought to be able to fix--unless u have admin access--(ie u r an admin'r of the of the network not that u are in the council's admin dept--i know it sounds confusing--every system has "users" and at least one of these has "admin status" and can make changes to the system)--there is not a lot u personally can do--as fender 62 says a virus , adware and spyware scan is required to find what is hijacking the home page and then clear it out--after that has been done-- download and install home page guard from
http://www.shareup.com/Npust_Homepage_Guard-do wnload-9758.html
this allows u to lock the home page and it cannot then be hijacked--cheers