that if you have an upgrade version of w7 you need a full previous version of windows to hang it on. Assuming you meet the upgrade criteria, you will only have a licence for one full OS, and if you are using w7 then you cant legally use the full xp as it is tied in with the w7 installation.
Obvious way round this problem is to buy a full install version of w7.
Thats my take on it anyhow.
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