re-use XP

My current computer line up

1. 2007 box with VISTA
2. 1999 box originally with Win98 but changed in 2005 to XP using a retail XP upgrade disk.

My proposed future line up

1. 2007 box with VISTA
2. 1999 box scrapped
3. 2008 iMac with OS X and XP dual boot.

Question

Am I right in thinking that the upgrade disk will not install on the iMac (because it won't find a qualifying product) even though there is a full OS on the disk?
If so is there any method of converting the disk, perhaps by buying a new license, or is the only option to buy another full disk?

Andereida

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