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#11
You only need to have the qualifying XP or Vista on hand and not use it again during the period it is qualifying to use Win7 Upgrade version. It does not need to be installed.
When installing Upgrade version to a wiped or clean HD, leave key blank during install then after install use one of the workarounds in the tutorial to activate at Computer>Properties link.
Easiest is the Registry edit; set a System Restore point in case you goof.