Boot with the XP (pick one) CD in the drive. When prompted, choose to remove all partitions and format the drive in NTFS. You must format the drive from the CD. Then create a new partition and install XP. Hopefully you have all the drivers for video, audio, modem, NIC, etc. as XP does not have them.
A friend gave me Pentium4 Gateway computer that had some sort of virus/spyware to the effect it would lock up as soon as Windows ME would load. So I decided to install windows XP. It installed just fine, then I realize I don't have a product ID key for XP Home Edition, so quick thinking me scours the house and finds a Windows XP Pro disk, with a product ID, I figure I can just upgraded. I slip the disk in and then it restarts then begins to say profmap.dll ( it would say a couple of other dll. files also, I tried reinstalling the update several times) will not copy. Then it asks me to either skip or retry ( I have done both.) I have tried format c: then reinstalling Windows XP Pro several times but it always says it can't copy a .dd files ( it's not the disk, I have 3 copys of WInXPpro and they all look unscratched to me and I have tried them all) Right now I have formatted C: and the computer comes on then just past the Gateway screen is tells me "NTLDR" is missing Press CtrAltDel. Should I just trash this machine (it's a lot nicer than the P3 I am on now) or can it be repaired?