Run a customized installation of 2007 and make sure to go through all
tabs in the setup wizard before pressing Next. In one of those tabs you
can choose to keep all previous versions. That will keep 2003 on your
computer and you can run it side-by-side with 2007. Note that there can
always be only one version of Outlook installed at any time. If you
install Outlook 2007, 2003 will be uninstalled automatically.
The rule for dealing with two Office versions is: install the older one
first (2003), then the newer one (2007). Uninstall in reverse order, so
2007 first then 2003. If you want to reinstall 2003 (e.g. because you
want Outlook 2003), remove 2007, reboot, reinstall 2003, reinstall 2003.
Patrick Schmid