If you want to have a dual boot system with Windows XP and Windows Vista, then you must first install Windows XP and then Windows Vista.
Windows Vista uses a newly designed boot loader and if you install Windows XP after Windows Vista, then it’ll overwrite Windows Vista boot loader and you’ll not be able to boot into Vista.
If you have installed XP after Vista and you are not able to boot into Vista, then follow these simple instructions and you’ll get the Vista boot loader back:
1. Download “VistaBootPRO”. Its an excellent software which can reinstall Vista boot loader within a few seconds.
2. Install it and run the software.
3. Now click on “Bootloader” button as shown in following screenshot:
4. Select “Reinstall the Vista Bootloader” option and click on “Configure It” button.
5. Thats it. It’ll install the Vista Boot loader. You can also edit other settings like boot loader timeout, default boot entry, boot entry labels, etc.
Google to download “VistaBootPRO”. Its not a freeware.
You can also repair Vista boot loader using Vista Setup DVD but the above mentioned method is the easiest method and works even you don’t have the CD/DVD.