The easiest way to replace it is to boot to the XP install CD, follow the setup screens to the one where your select the partition/drive for install. Format the drive/partition that currently has W7 on it, and install XP there.
I would like to remove Windows 7 pro and replace it with Windows XP Home Edition Upgrade which I have a copy of ?
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[#0005ff]Just to supplement Treefrog's advice, I suggest you check the manufacturer's website for compatibility first just to be certain they provide drivers for XP if the machine was designed around W7.[/#000ff]