Install SQL Server Instance only using Command line Arugument

I am developing windows application using C# and SQL server 2005. For customers i am creating installation package using install shield. Currently the insatallation package will install SQL server express edition and create the our own intstant name.(if the system has SQL,then also it will install the sql and create the instance).Till now no problem we faced.   But one of our customer told that they have used other software which have already SQL. So no need to update or install the SQL server components. We should create only instance name only(they are worring ,during the SQL updation , it may crash existing application).
                                            Means that if Systam has alrady SQL ,then only create instance name.If system does not hav SQL then Install SQL and create instance name.
                                                Please guide me to solve this issuse.