What user account is being used for the BE services? Is it the same account as was used in 9.1? Did you type in the account during install or did you accept the account that appeared at the services prompt? User account vs admin account? Workgr...
The original error occurred because you uninstalled BE 9.1 (keeping the BE data) before installing (upgrading to) BE 10.0. Why did you uninstall 9.1 before trying to "upgrade"?