How Do I Dual Boot With XP And Vista?

Have atleast two partitions, each should have plenty of space on, atleast 30 GB i would say. Install XP on one first (if it's not already), then install Vista on the other. Now, you should download a program which will overwite your bootloader and allow you to choose between the two (EasyBCD, but don't use it unless you know what you;'re doing.)