Sorry for the trouble. I tried installing DigDB on Excel2007 on vista and everything works just fine. I need your help to see if I can recreate the error you saw, then I can find a fix.
Here is how I install DigDB on Excel 2007.
I log on to my machine as an admin, and I download the DigDB setup file and I install it.
For other users to use DigDB on this machine, I log on as a local user, and then I click the big round office icon button located at the top-left corner of Excel, then find and click "Excel options" button, then in the left column, find and click "Add-ins", then in the right pane, towards the bottom of the pane, find "Manage" label followed by a dropdown which defaults to "Excel add-ins", click "Go..." button to manage add-ins, then you see the old familiar add-ins checkbox. Find Digdb in the list, check it, and that will add DigDB to the local user's Excel menu.
I tried to log on as a standard user and run the DigDB setup, but found as a standard user I don't have the permission to run the the setup.
Is that how you install DigDB for your users?
Also, I wonder if the error you saw could be caused by security settings.
Go to Office icon->Excel Options...->Add-ins->Manage-Dropdown to "Disabled Items"-Go... do you find Digdb in the disabled items? If yes, enable it.
Go to Office icon->"Excel options", go to "Trust Center", then click "Trust Center Settings...", under "Add-ins", what option do you have? Under "Macro Settings", what option do you have? Try remove all restrictions and see if works.