Hi,
First please remember Windows Vista will not start after a dual-boot Windows XP is installed. But we can refer to the following article to make Windows Vista work again.
http://support.microsoft.com/kb/919529/en-us
As I know, some kinds of computers such as ASUS only support Vista now and we cannot install XP on them. Therefore, I suggest you to contact your computer manufacturer first to confirm if your computer supports XP.
If it supports XP, please boot from the XP installation disk, format the partition where you would like to install XP and then install XP according to the directions. After XP is installed, you will be able to enter XP but not Vista. Try the method below to rescue your Vista:
http://support.microsoft.com/kb/919529/en-us
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