This threading model is not recommended for components to be called by ASP pages. Single-threaded objects are not suited for applications that need to accommodate a large volume of users simultaneously. Using these objects can result in a deadlock when multiple users try to access the object simultaneously. All calls to Single-threaded objects are serialized through one thread, the COM Main thread. Calls to these objects from other threads are marshaled to this thread. Only one thread at a time can enter a set of Single-threaded objects, and it must be the same thread each time.
Finally, Single-threaded objects work in the context of the SYSTEM account and have all permissions associated with the SYSTEM account. In most cases, this is not desirable for Web sites.
If you are developing with Visual Basic your component will be single-threaded unless you specifically set it to Apartment on the General Project Properties page.