Creating a dual-boot installation is in itself no problem, but as a rule the oldest OS has to be installed first i.e. first XP and then Vista. XP's boot manager doesn't recognize Vista, but does recognize older versions of Windows such as 98 and 2000. Similarly, Vista's boot manager recognizes all the older versions including XP. I'm not sure whether it is possible to install XP AFTER having installed Vista- better check this first before you start work.
You can install several different versions of Windows on one PC, as long as you install the oldest first and the most recent last: 95>98>2000>XP>Vista.
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