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What version of XP Pro is your upgrade CD? I think if you can do a clean install of XP with it when the installer begins, as I recall it will just ask for the Win 98 CD to be inserted at some point during the install to confirm ownership. I do not remember exactly since I only upgraded one '98 machine years ago.

If it is an older version of XP you can slipstream the CD into XP SP2/3 with nLite, but you need a computer running Windows to do this. There are manual methods to slipstream too I would try googling it. However the only methods I know require Windows to be installed.
 
What version of XP Pro is your upgrade CD? I think if you can do a clean install of XP with it when the installer begins, as I recall it will just ask for the Win 98 CD to be inserted at some point during the install to confirm ownership.

This method should work and is the preferred method when using an upgrade disc.
 
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