An upgrade to Windows XP Pro is a one-way ticket with no
uninstall option.  You'll have to reformat the drive and reinstall
XP Home "clean".

Read your Windows XP End-User License Agreement:

Start > Run and type:  WINVER  , and hit enter.

Also, open XP's "Help and Support" and type:  EULA
and click on "Questions and Answers about the
End-User License Agreement".

The Windows XP CD is bootable and contains all the tools necessary
to partition and format your drive.  Follow this procedure and allow
Windows XP to partition and format your drive:

1.    Open your BIOS and set your CD Drive as the first bootable device.
2.    Insert your Windows XP CD in the CD Drive and reboot your computer.
3.    You'll see a message to boot to the CD....follow the instructions.
4.    The setup menu will appear and you should elect to delete the existing
      Windows partitions, then create a new partition, then format the primary
      partition (preferably NTFS) and proceed to install Windows XP.

5.    Windows XP Clean Install:
      http://www.winsupersite.com/showcase/windowsxp_sg_clean.asp

6.    Accessing Motherboard BIOS
      http://www.michaelstevenstech.com/bios_manufacturer.htm