My home page does come up

Question

i have a toshiba p300 since august 2008 no problems until now my home is set as igoogle an internet is working fine also got norton 360 which i have done a full scan no viruses at all. when internet explorer opens my home page does not come up nor google or yahoo all i got is microsoft security center saying you been attacked by a antispyware or virus download now! plz can u help me..............