Condition:
You have installed Vista and now you want XP to be on the second boot, and when you do, your Vista is not detected anymore, so you have to choose between re-format your drive and then install it again, or you repair Vista and it does not matter.

But you can run both in a dual boot without having to reformat.
Seetlah your Vista install, you install XP as the drive x:> (x is the drive of your choice, it could be d:, e:, etc..)

After you restart the computer, boot to Vista it does not appear, do not worry.
Next please you boot from the CD / DVD Vista, select repair the installation, now booting from the Vista works, but XP does not boot up, do not worry once again.

Place the CD / DVD drive your Vista in Vista (after entering the desktop) and go to the Start Menu, All Programs, Accessories and right click on the Command Prompt and click on Run As Administrator

Now follow some of the following commands:

Use Bootsect.exe to restore (restore) the MBR and boot from the Vista code that transfers control to the Windows Boot Manager program. To do so, enter the following command at the command prompt:

Drive: \ boot \ Bootsect.exe-NT60 All

In this command, Drive is the drive where the installation media is Windows Vista.

Use Bcdedit.exe to manually create entries in the BCD Boot.ini file for the earlier versions of OS Windows. To do so, enter the following command at the command prompt.

Note that you need to look at it again in the following command is that the Drive is the drive where Windows Vista installed.

Drive: \ Windows \ system32 \ bcdedit-create (NTLDR)-d "Description for earlier Windows version"

Note that the words "Description for earlier Windows version" you can change the text or any text. For example, you write it with "Windows XP" or "Windows Server 2003"

Drive: \ Windows \ system32 \ bcdedit-set (NTLDR) device partition = x:

Note that x: is the drive letter for the active partition.

Drive: \ Windows \ system32 \ bcdedit-set (NTLDR) path \ ntldrDrive: \ Windows \ system32 \ bcdedit-displayorder (NTLDR)

Restart your computer.

Good luck!