The screen shot there simply shows how that section looks in Vista in the msconfig for setting the timeout seen on the right side there. If I was currenly dual booting XP instead of seeing both as stand alone OSs you would also see the additional entry for the older version there with the option for setting that as the default OS to load.
The one thing to remember about dual booting the two versions however is the loss of Vista's own restore points. There's a work around to see those preserved however in the guide written by an MS MVP seen at
John Barnett's Windows Vista Support: Prevent System Restore Points Being Lost When Dual Booting With Windows XP