Initial conditions:

1. C:\ drive to install Windows 7,
2. drive for example F:\ ready to install Windows XP.

Problem: If you install Windows XP operating system on a Windows 7-based computer, Windows 7 no longer starts. In this case, only the Windows XP operating system starts. Solution for dual boot Windows 7 with XP:

1. Start Windows 7 System Recovery Discs and repair existing Windows 7 instalation (if you don't have any, you can download Windows 7 System Recovery Discs).
2. As administrator copy file F:\ntldr and F:\boot.ini into root C: (because to run a version of Windows operating systems based on Microsoft Windows NT, these files are necessary).
3. Edit file C:\boot.ini

   [boot loader]
   timeout=30
   default=multi(0)disk(0)rdisk(0)partition(3)\WINDOWS
   [operating systems]
   multi(0)disk(0)rdisk(0)partition(3)\WINDOWS="Windows XP" /fastdetect /NoExecute=OptIn

   Note: You can change bold written text, partition number depends on the partitioning:

   default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS

4. Restart the computer.

Post-installation verification: Click Start, click Accessories, right-click the command-prompt shortcut, and then click Run as Administrator. Use Bcdedit.exe for verification entry in the BCD Boot.ini file for the earlier version of the Windows operating system, more http://support.microsoft.com/?scid=kb%3Ben-us%3B919529&x=18&y=10