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It is always recommended that you keep the packaging which has the key but it happens often that a re-install is needed and the key is lost so they should not give you too much trouble on the phone getting a new key. Here is the microsoft knowledge base article with this information. http://support.microsoft.com/kb/2548260 Click on the link that says "You purchased Office at a retail store"
They do sometimes have a small charge for the replacement however it is cheaper then having to purchase the whole program again.
I am sorry there is not an easier way.
If you need further help please do not hesitate to reply back and let me know.Thank You