Although I would mostly agree with Niclo,I would strongly advise against
an upgrade of any kind.
You need to boot to the xp cd and have it wipe the drive clean
(after you have saved everything you want).
Lastly what Niclo says about not being allowed to use the XP from another computer is true .
But you said the computer originally came with XP so if thats the license your using then it should be fine.
You dont do it from within windows.
You must boot to the XP cd and when it is installed it will require you to activate it. SO be sure you use the key that computer was originally sold with.