I don't think you can change the default position of the comment.

But you could use a macro to reposition them each time the workbook opens.

Debra Dalgleish has some code that may help you get started at:
http://contextures.com/xlcomments03.html#Reset

You could modify Debra's code to go through each worksheet and name it Auto_open
so that it runs when the workbook opens.

If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm

Hans Feitz wrote:

If a cell with a comment assigned to it is towards the edge of the (RH) side
of the window, the comment box will only be partially visible. Can the
default position of these boxes be changed to f.i. LH side of the cell? I
know that during editing the text box position can be changed but that is not
reflected permanently afterwards.

--

Dave Peterson