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:
?:\boot\directory of the Win7-DVD (with
?being the letter of your DVD drive)
run the following command: (with
C: being the drive where Windows 7 is installed)
bootsect.exe /nt60 C: /mbr