Here is a excerpt from a article by Scott Dunn
It is a much longer article but I just provided what you needed.
Search the Vista forum if you want the entire article. I have posted it several times.
W7 works the same as Vista
Step 7. Your system now boots to XP, so we'll need to do some fixing to set up a boot menu. Insert your Vista DVD and restart the computer from it. Click Next in the first screen.
Step 8. Don't click Install when prompted! Instead, click Repair Your Computer in the lower-left corner.
Step 9. When the System Recovery Options dialog appears, make sure Microsoft Windows Vista is selected and click Next. In the next dialog box, select the Command Prompt option at the bottom.
Step 10. In the command-prompt window, type the following commands and press Enter after each one:
bootrec.exe /fixMBR
bootrec.exe /fixBoot
Step 11. Close the command prompt and click Restart.
Step 12. When your computer has booted into Vista, click Start, type cmd.exe, and press Ctrl+Shift+Enter to make the command window open with elevated privileges. Click Continue, if prompted by User Account Control.
Step 13. Type the following commands in the command window, one at at time, pressing Enter after each one. After each command, you should get the response, "The operation completed successfully." A response of, "The specified entry already exists," is OK, too. If not, retype your command to make sure you've entered it correctly. If Vista is installed on a drive other than c:, change the first command below to use the proper drive letter. The curly braces around {ntldr} in each command must be typed exactly as shown:
bcdedit -set {ntldr} device partition=C:
bcdedit -set {ntldr} path \ntldr
bcdedit -displayorder {ntldr} -addlast
bcdedit -set {ntldr} description "Microsoft Windows XP"
That's it! The next time you restart your system, you should be see a prompt that will let you choose between Vista or XP. Select the one you want and press Enter.