In short: you can't do that.
Longer explanation: Excel doesn't have the command line option to specify password for a protected file. If it had, you could change your batch file to "C:\Program Files\Microsoft Office\OFFICE12\excel.exe" /w "MyPassword" MyFile.xlsx
But it hasn't (see http://office.microsoft.com/en-us/excel/HA101580301033.aspx).
Also you may want to post such questions on Office-related forums, I'm sure you'd get the answers much more promptly. This forum is about Windows Academic Curriculum.