i want that a qv document start loading on success of an external ETL job (SISS). Is there any way to do this in a simple manner using SISS itself or windows scheduler?
Yes, you can do as described here http://community.qlik.com/docs/DOC-2650
EDX is one option. Depending on where the SQL, QV servers and data are, an alternative might be to use an Execute Process Task in your SSIS package similar to this ...
EDX - SSIS - QV11
Retrieving data ...