Yes, if you have Office XP )both CDs and product key), you can reinstall
it. I believe the recommendation is that you first uninstall Office 2007,
then reinstall Office XP, then reinstall Office 2007, specifying that you
want it *not* to replace the existing version. As you obviously know, you'll
need to choose one version of Outlook; if you don't use it, you might as
well opt for Outlook 2007, which I'm told is a vast improvement, but it will
not be installed by default if you install Office 2007 side-by-side with
another version.
For some tips on getting the two versions to coexist peacefully, see
http://word2007bible.herbtyson.com/2007/04/15/word-2007-and-word-2003-coexisting/
and "Information about using 2007 Office suites and programs on a computer
that is running another version of Office" at
http://support.microsoft.com/?kbid=928091