MSFT writes how at http://support.microsoft.com/kb/302822
Sucessfully upgraded from Office 2003 to Office Pro 2007 on an older desktop 2 years ago. Recently bought a new desktop, transferred files and then tried to install the upgrade version of Office Pro 2007 on new PC. Asked that I insert the Office 2003 install disc, which cannot be found. Anyone know how we can get the Office Professional 2007 Upgrade version to install in the absence of the original Office 2003 install disc? Running Windows 7 Pro 64 bit on new PC.