Well if you didn't have SP1 installed along with XP when you first put in your HDD, you might have partitioned the HDD with just the size XP could first see. You should go to Disk Management and see if you have some unpartitioned space there. You can find Disk Management under Computer Management, which you get to by right clicking "My Computer" and select "Manage". If you can't see the total space there, your BIOS might not support large hard drives. In that case I will need the manufacturer and model name of your mainboard.