Theres some things I need to know to better tell you.
Do you have an OS installed already? Vista or XP?
I can tell you how to do it either way. I just need you to tell me what is already installed. If nothing is installed then install XP and then Vista.
If Vista is installed 1st then you can use your Vista installation DVD to resize your partition. Then install XP. You wont be able to boot into windows after that so you will need to use your Vista DVD again to do a repair on the boot record. (not a full installation). Then you wont be able to boot into XP but theres a free program that you can download and use to rebuild the XP boot record and then you are set.
Go here to see how to do it step by step. Choose what OS to what OS in the drop down menu and it will give you the correct directions:
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