See lib/blocklib.php, function blocks_get_default_blocks(). It has an optional parameter that does what you need. I 'm afraid there is no easier way right now, although I would like very much to have one. But there is no time...
Thanks for pointing me to that function. That helps. And for changing the existing courses I found it convenient to use phpmyadmin to do it straight in the database.
In the long run it might be a good idea to allow the administrator to specify a course that should be used as the default. Or even better: make the block administration page look just like a course homepage so that the default can be set right there by moving the blocks around.
How could i do want you said. I m a roockie.
unless you feel comfortable with messing around in your moodle database you probably don't want to try this. If however you do want to try, arrange the blocks the way you like in one of your courses. Then go to the course table in your database and copy the entry in the 'blockinfo' field for that course into all the other courses that you want to have the same block layout. To do the later you need to use the sql update command. Are you still sure you want to do it? Filing a feature request at http://bugs.moodle.org for a user interface for this might be the safer alternative.