How to enable default homepage change??

Hi, my pc is Window XP Professional and I uses IE. The problem I am having right now is, somehow the home page of my browswer was set to a junk website without my knowledge. I tried to change the default home page back to my choice by going through my IE's Tools --> Internet Options. But I found that I am no longer allowed to change this home page because it's been disabled (all fields in Home Page section are greg now). Anyone please help me - how I can change my default home page back ?? Thanks!!