Excel 2003 and 2007 running - how to handle two different personal.xls and xlsb workbooks

Need to run Excel 2003 and 2007 simultaneously on two computers, one running XP and the other running Vista. How can I set up the Personal workbooks that will contain different macros? I can't put them both in a Startup folder because both versions will start. Thanks.

More information. If I place Personal.xlsb, the macros that I use for Excel 2007 workbooks, in the folder that it's supposed to go in, which is 'C:\Users\myname\AppData\Roaming\Microsoft\Excel\XLSTART', then when I start Excel 2003, it loads its own 'Personal.xls' macro workbook, but the 'Personal.xlsb' workbook also gets loaded. I don't want that. Where can I put the 2007 Personal.xlsb workbook so that it doesn't load when I start Excel 2003? Or is there another way to prevent both from loading? Thanks.