Use "USMT and Easy File Transfer" to migrate your system to Windows Vista if profiles are outside the system drive
When you are trying to upgrade the system to Windows Vista, and the profiles are outside the system drive (example, profiles are in Drive D:), you will see the Upgrade option is grayed out, only ‘Clean Install’ is available.
This behavior is expected and by design, Vista upgrade is not designed to support profiles outside the system drive. At this moment, you need to use “USMT and Easy File Transfer” to migrate your system to Windows Vista
Reference:
User State Migration:
https://www.microsoft.com/technet/desktopdeployment/userstate/default.mspx
Zero Touch Installation (ZTI):
https://www.microsoft.com/technet/desktopdeployment/bdd/2007/ZeroTouch_7.mspx
Desktop Deployment Center - BDD 2007:
https://www.microsoft.com/technet/desktopdeployment/default.mspx