Unable to defragment hard drive WinXP Pro

I have not been able to defragment completely the hard drive on my relatively new custom built computer. The defrag process completes, but there is a large percentage that stays fragmented. I have Win XP Pro installed from a full install disk and fully updated. I have the same version installed on two other older computers, and their HDs defrag completely with no problems. I have tried defragging with both Window’s built in defragger and with EaseUS Home version. Neither worked. Anybody have any ideas? Note: I just recently noticed that Windows had only allocated about ¼ of my HD during original formatting leaving ¾ “unallocated.” I merged the unallocated space using EaseUS and now have access to the entire drive on one partition (C .

Thanks much for trying to help me out with this
Pete