I have my Business Objects in a separate DLL from my Application Exe. One of the problems I've always had in installing Updates , where if there are any users actaully using the application, they need to logout before the update is done (as the .EXE will be In Use). A lot of the time, the only reason for an update would be a change in the business rules, and the Application.exe would not have changed, but the BusinessRules.DLL would have.  In my innocence, I thought that this restriction might not be there for the .DLL, but having tried to over-write a DLL with he application in use, it would not let me. If somebody could confirm that, no you cannot overwrite a DLL if the exe that uses it is open, or perhaps there is another way of over-writing a DLL if the Exe is in use. It would make life a lot easier for updates if even the Dll's could be over-wriiten (if not the Exe).