Windows – Formatting of an XP/Vista dual boot machine now unable to boot up XP

Install EasyBCD to reconfigure your bootloader and delete XP from the startup.

Then delete all the Windows XP data (Documents & Settings, Program Files, Windows etc), in case you have Windows XP on a different partition just format the entire partition AFTER taking it from your bootloader.

Since Vista was installed first the boot files will likely be located on C:.

If you want to format C: win 7 won't let you from inside Windows since it will cause boot problems. Easiest way to perform the format is to boot from your Windows 7 media and then start the install. At the harddrive selection screen you can format C: and then cancel the setup.

After you have done this Windows 7 will not boot. To fix the issue you boot from the Windows 7 media again and instead of Install now you select repair at the bottom left corner. At the repair menu you select automatic startup repair and let it finish. This should restore your boot files and give you a working system again.