Why not just save your 2007 file (.xlsx) as a 2003 file (.xls), but do not forget saving into a backward comparability may render some of the features only avaliblie in 2007 redundant and your 2003 workbook may not work as you thought it would, but you will still have the original 2007 file if any thing goes wrong
Why did you post a question about conversion between 2007 and 2003, get an answer and then edit your post and delete the question. The whole purposes of this board is to get help and then allow others to share. I may not have posted the best answer and others may have shown one