Ok, tried that too, still the same result.
A bit more detail on the error messages.
It first comes up with an error window that says "Dungeon Siege III has encountered a problem and needs to close. We are sorry for the inconvenience." - standard Windows program failure stuff with "send error report" and other useless crap that Microsoft will never do anything about. I suppose I could list the full content of the error report if anyone thinks it will help, but I really doubt it.
Anyway I close that, and another window pops up. It's headed "Microsoft Visual C++ Runtime Library" and the body message is:
And just an "ok" button. I press that, and another error window pops up. This one is headed "Dungeon Siege III: Dungeon Siege III.exe - Application Error", the body of this error message is:
And another "ok" button, and when I press that one it finishes giving me error messages and still no sign of the game that has been paid for.
It's still failing file validation.