I have developed Logon Tweaker Vista . But sometimes i experience problems while changing login screen

Logon Tweaker first takes ownership of all files . Then it copies the original dll from system32 to program data folder and updates it's resources and saves it there. Then the imageres.dll in system32 is moved to desktop and the file which we modified at program data folder is copied at it's location

But sometimes it failes

Access is denied error 5 when i try to replace,move or delete the mageres.dll file

Why so

I am using updateresource,endupdateresource funtions and pinvoke calls