Registry editor has been disabled by the administrator

I got this problem when i open IE, when the popup comes... IE have a soWar browser attached ... and appear redtude homepage which is a pornographic site ... so shock since i put yahoo.com as my homepage ... i tried to change back my yahoo homepage but still this redtude always appeared .. so i ask someone what to do.... and told me to go to my registry.. but when i type regedit... popup comes out ... registry has been disabled by the administrator i am the only one who uses this pc no one else ... what can i do since my task manager , my registry and some of my pc security have been tampered by this ... when i install my anti virus it runs automatically but not it isnt ... my operating is window xp prof sp2 ...what shall i do .. do i have to reformat again this pc? i am not an expert about pc only a very limited knowledge ...any idea how to fix? thank you and more power ..God bless