If you installed Windows XP after Windows 7, you're left with the XP-MBR on your drive and thus only XP is booting.

If you happen to have hardware, which produces the Code 5 error upon trying to boot from the Win7-DVD, you can do the following to restore the Win7-MBR from within XP:

  1. boot into XP and insert the Win7-DVD
  2. open a Command Prompt
  3. navigate to the ?:\boot\ directory of the Win7-DVD (with ? being the letter of your DVD drive)
  4. run the following command: (with C: being the drive where Windows 7 is installed)

    bootsect.exe /nt60 C: /mbr
  5. reboot and use EasyBCD to add Windows XP to the Win7-Bootloader