What local application is launched depends on the user's browser settings,
so for the most part you cannot control what application is launched. If you
want the file to be downloaded (which is what I am guessing you are asking
based on the subject), you will need to use a few methods of the Response
object. Here are some of the most important ones:

Response.ClearContent()
Response.ContentType  (I have included a function below to help determine
the value for this)
Response.AddHeader("content-disposition", "attachment;filename=" &
txtFileToDownload.Text)  (see the documentation for more details on the
parameters for this method)
Response.WriteFile()
Response.End()

Here is a function to help you determine the value for Response.ContentType:

Public Function GetMimeType(ByVal extension As String) As String
   Dim regkey As Microsoft.Win32.RegistryKey =
Microsoft.Win32.Registry.ClassesRoot.OpenSubKey(extension.ToLower())
   If regkey IsNot Nothing AndAlso regkey.GetValue("Content Type") IsNot
Nothing Then
       Return CStr(regkey.GetValue("Content Type"))
   Else
       Return ""
   End If
End Function

Hopefully all this will help you accomplish whatever you needed. Good Luck!