New Post: Gettings a exception Error (15,8):userId:

That error usually means that the TaskDefinition.Principle.UserId or the userId parameter of the RegisterTaskDefinition has been supplied an invalid user account name.

To answer the second question:

Using TS As New TaskService()
   If TS.FindTask("MyTaskNameHere") = Nothing Then
      ' You don't have that task
   End If
End Using