Office 2007 installed Outlook 2007. As two versions of Outlook can never
exist on the same computer, your Outlook 2003 was removed.
Before you go on, make sure you have your Office 2003 CDs.
I'd suggest to completely remove Office 2003, reboot and then reinstall
it. If that doesn't work, installing 2007 as an upgrade to 2003,
rebooting, removing 2007, removing 2003 and reinstalling 2003 might do
it.
Alternatively you can try to use Windows Safe Mode while 2003 is removed
and delete some files manually (namely the ones Outlook should give you
error messages about).
Patrick Schmid