Situation:

1. You have a system where Windows Vista was already present.
2. You installed Windows XP into it to make it a dual boot system.
3. Now when you boot up, only Windows XP is loaded and not the Vista.
4. You want to boot the system with the Vista OR Dual boot with both Vista and XP now. So, what to do?

Solution:
Why previously installed Vista cannot be loaded after installing XP is because When you install XP over a pre-installed Vista, the XP setup overwrites some critical files like MBR, the boot sector, the boot files, etc. So, in the next startup, the system can only locate the XP loader and nothing else. Therefore, our job now is to create a new boot loader for Windows Vista so that in the next restart, the system finds the loaders for both XP as well as Vista.

Okay, assuming that you have right now loaded XP in your system, let’s start the process:

1. Insert the Vista DVD (or the Vista backup DVD that came along with the System).
2. Open command prompt and type the following command line:
   <Your DVD Drive>:\boot\Bootsect.exe /NT60 All
3. Next type in the following command line
   <Your Vista Drive>:\Windows\system32\Bcdedit /create {ntldr} /d “Windows XP Professional”
4. Then run one by one the following command lines:
   <Your Vista Drive>:\Windows\system32\Bcdedit /set {ntldr} device partition=<Your XP Drive>:
   <Your Vista Drive>:\Windows\system32\Bcdedit /set {ntldr} path \ntldr
   <Your Vista Drive>:\Windows\system32\Bcdedit /displayorder {ntldr} /addlast
5. Restart the computer

If everything goes fine, you should get an option to load either XP or Vista now.

Alternative:
If you think command line thingy is very confusing, you can try out with EasyBCD. EasyBCD is a software that will help you easily manage your Boot Loader files. To proceed with EasyBCD, you can restart your system after STEP 2 in the above procedure. This time, your Vista will load. Then download EasyBCD and create a new boot file for your XP. I hope you will find EasyBCD very easy. But if you need help on that, I am always ready to reply your comment. Have a great day.