1. Invoke a command prompt with Administrator priviledge. If you don’t know how to do it. Please follow the steps in this post. The instructions are for Vista, but they are the same.
2. Type the following to list the current boot manager settings.
C:>bcdedit
Windows Boot Manager
--------------------
identifier {bootmgr}
device partition=C:
description Windows Boot Manager
locale en-US
inherit {globalsettings}
default {current}
resumeobject {59cad9f2-de83-11dd-9d9e-c59ac6c2613b}
displayorder {current}
toolsdisplayorder {memdiag}
timeout 30
Windows Boot Loader
-------------------
identifier {current}
device partition=C:
path Windowssystem32winload.exe
description Windows 7
locale en-US
inherit {bootloadersettings}
recoverysequence {59cad9f4-de83-11dd-9d9e-c59ac6c2613b}
recoveryenabled Yes
osdevice partition=C:
systemroot Windows
resumeobject {59cad9f2-de83-11dd-9d9e-c59ac6c2613b}
nx OptIn
3. Type the following to create a Boot Loader based upon the default boot loader.
C:>bcdedit /copy {current} /d "Vista"
The entry was successfully copied to {59cad9f6-de83-11dd-9d9e-c59ac6c2613b}.
4. Type the following to change the device to D: drive. Note the identifier is the one generated by step 3. This id is different for every boot loader. You have to replace it with the actual output when you create the boot loader.
C:>bcdedit /set {59cad9f6-de83-11dd-9d9e-c59ac6c2613b} device partition=D:
The operation completed successfully.
5. Type the following to change the osdevice to D: drive
C:>bcdedit /set {59cad9f6-de83-11dd-9d9e-c59ac6c2613b} osdevice partition=D:
The operation completed successfully.
When you are done, reboot the machine and you’ll see the option to boot to Vista.
CREDIT TO amida168 : http://www.kombitz.com/2009/01/09/how-to-add-vista-to-windows-7-boot-manager/