Windows system, when use Teradata SQL Assistant and TSET connect to a Teradata Database, it alerts "no default driver specified" - response (1) by MikeDempsey

I think something is missing from this question.
I assume these are 2 completely unrelated statements:
  SQL Assistant gives an error message
  TSET gives an error message
since you cant use TSET from within SQL Asistant.
It sounds like you have not created any Teradata ODBC data sources. (Use ODBC Administrator to do this)
If that's not the case then it sounds like something in your registry (related to the ODBC manager) is corrupted. 
SQL Asistant does not use 'default' drivers even if they are created. It expects you to provide a data source for your connection.