Posted in another excel newsgroup 7/11/08 but no solution:

I'm using Office Home and Student 2007 on a Vista Home Premium machine. I'm
unable to change the default save extension from xlsx to xls. I can do it
on an individual file by file basis but I would like to have the program
always save the files to a default extension of xls. I've changed the
setting in Excel Options and the setting "sticks" but the program continues
to create and save as xlsx.

I also have the same problem in Word. Any help is much appreciated!

Thanks in advance.