Excel 2007 default file format

Hello all. I have Excel 2007 and I want to change the default save option to xlsx or xlsm. When I do this it works but if I close and re open Excel, it changes the default back to xls. How can I solve this? Thanks

Maybe this will help:

http://technet.microsoft.com/en-us/l...ffice.12).aspx

or

http://www.mydigitallife.info/change...up-policy-gpo/