Dont know exactly how to do the behavior you describe but two options
are :

1) If you want excel 2007 to be used just open the file, but if you
want 2003 to be used open excel 2003 and then open the file (doesnt
have to be from excel 2003, can be from windows explorer) if you have
both 2003 and 2007 open at the same time whichever one is active will
be where the file opens

2) Set your default for .xls extension with windows