I don't have Office 2007 but you should be able to do it the same way as previous versions by changing your normal.dot template.
First make sure that you can see hidden files. (My Computer, Tools, Folder Options, View tab, then select 'Show hidden files and folders'.)
Find normal.dot, by searching if necessary, but the default location for XP and Office 2003 is 'C:\Documents and Settings\'your username'\Application Data\Microsoft\Templates'. If you have any macros or have made changes to your toolbars this would be a good time to save a backup of normal.dot as this is where such information is stored.
Start Word, click 'File, Open', navigate to normal.dot and open it. Turn on 'track changes' and click 'Save' then close Word. Next time you open it your new normal.dot template will be used with 'track changes' turned on by default.