You have to choose between Outlook 2003 and Outlook 2007 because you can
only have one or the other, but you can have both 2003 and 2007 versions
of the the other Office applications. If you want to keep the 2003
versions of Word, Excel, etc., alongside the 2007 versions, then choose
a Custom installation of 2007, and choose to keep the 2003 versions in
the Installation Options tab.
As a precaution, back up Word 2003's normal.dot template and any custom
templates you created, autocorrect list, and custom dictionary before
you install the 2007 version. See
http://word.mvps.org/FAQs/General/FilesToBackup.htm and
http://word2007bible.herbtyson.com/2007/04/15/word-2007-and-word-2003-coexisting/
and
http://support.microsoft.com/?kbid=928091 "Information about using
2007 Office suites and programs on a computer that is running another
version of Office" for more information.
