When you run or boot the Windows 7 DVD it scans your system to see any other installed OS, which will allow the use of Upgrade version key upfront.
If it doesn't see another OS, it will reject the key but allow you to install anyway to activate later.
Once you do this, after install you must do a quick workaround to install to a new or wiped HD. The easiest is the Registry change - set a Restore point in case you goof: Clean Install with a Upgrade Windows 7 Version
You can audit the Product Key for Office off of the other HD if it still can be read by installing and running Belarc Advisor: Belarc Advisor - Free software downloads and software reviews - CNET Download.com
It should also be on the packaging if it is a legit copy of Office.