If we do it the other way around (Vista install followed by the XP install), we can only boot Win XP.

First of all, wee need to know that we can only have dual boot with Microsoft Windows Vista and XP if we follow the proper installation order: this means that we need to install Win XP first and then Win Vista. If we do it the other way around (Vista installation followed by the XP installation), we can only boot into Win XP. There will be absolutely no way to bring the Vista installation to the boot menu.

In order to have a proper install for dual boot, we can install Win Xp on the C:/ partition and the Vista on D:/ partition, just making sure there is enough disk space for the Vista installation (somewhere around 15 Gb of free space).

After both installations are complete, when starting the computer we will have a boot menu with two boot options: Windows Vista or Previous versions of Windows. When we wish to boot into Windows Xp we shall select the latter option. Besides, we can apply boot menu changes by running the Microsoft Configuration Utility (msconfig).