Was able to get around this Realport driver installation error in Win7 with the following:
Open Device Manager in Windows.
Select “Add legacy hardware” in the Action menu.
This gets you the “Add Hardware” wizard
Then choose the Install manually from a list option
Then pick “Multi-port serial adapters” even if your device is a single port Digi One SP
Follow the subsequent prompts and then reboot. Yes, it’s just like the old days.
Once you have your Digi instance under a new “Multi-port serial adapters” category in Device Manager then right-click and select Properties then Advanced then Properties again.
Then check the box to the left of the Port n
Then you can set the Com port number needed for Windows within the Serial properties tab of the dialog associated with the port you have highlighted.
Reboot again if necessary.
Fun and games!