One of the more frequent questions in the newsgroups in what to do when a password has been lost.  The following sites have products or add-in modules that claim to be able to break Excel passwords.  I have never used any of these products, so these are not recommendations or endorsements.   These URLs are provided only as a public service. 

www.accessdata.com

www.crak.com

www.lostpassword.com

www.elkraft.unit.no/~huse/xlpassword.htm

www.elcomsoft.com/ae97pr.html

home.telia.no/exceltips/

www.zuarin.de/sec_eng.htm

Remember, you should only recover password protected files that you have a right to view.  Opening up files that belong to others is unethical at best, and may be a violation of your license agreement or of copyright law.