have you setup a seperate partition for your XP install? If not you must do that first. You can do that from within vista.
You cant enter setup from Vista as you are running the 64 bit version and the XP is going to try to make it 32 bit.
Once you have the extra partition xp should find its new home.
You will then have to read this to get your boot order set up correctly since you are putting a older op system on after a newer one.
here is a article from the windows secrets new letter that should be of help.
Make your computer dual-boot Vista and XP
By Scott Dunn
It's getting harder to buy a new computer with Windows XP installed and — after Microsoft stops selling XP on June 30, 2008 — it will become nearly impossible.
Fortunately, you can have your XP cake and eat your Vista, too, by setting up your system to boot between the two operating systems.
Add an XP option to your new Vista system
If a new computer arrived in your recent past, or you plan to buy one in the near future, chances are it will come with Windows Vista installed. Although some manufacturers still give you the option of getting XP on your new system, that option is likely to dry up for most consumers later this year when Microsoft stops selling Windows XP.
If you're not ready to dive into Vista all at once, there is an answer: set up your Vista computer to dual-boot between Vista and XP.
In this article, I'll tell you how to make your PC give you a choice between Vista and XP every time you start up. In a separate article coming soon, I'll give you some additional tips that save disk space on a dual-boot system.
Although some sources, such as Computer Shopper Magazine, advise that you need an add-on product like VistaBootPro to dual-boot, you can accomplish the same thing without any additional software other than the two operating systems.
Before starting, make sure you have your Windows XP install discs ready as well as your Windows Vista DVD. In addition, it's a good idea to make a complete system backup before beginning an operation like this.
Once you've laid that groundwork, you're ready to go to work:
Step 1. In Vista, click Start, type diskmgmt.msc, and press Enter. Click Continue if prompted by User Account Control.
Step 2. Right-click a drive and choose Shrink Volume. Specify the amount to shrink, which in this case is the amount you want for your XP partition. At a minimum, you'll need around 2.5GB for XP Pro SP2. I suggest you select a larger partition to leave room for updates and other files that may need to be on the same drive as XP. Because I wanted a 5GB partition, I typed 5000 (representing 5,000 megabytes) in the available box. Click Shrink.
Step 3. Right-click the newly available area and choose New Simple Volume. Follow the steps in the wizard to assign a drive letter now, or wait until the next step. When prompted, check Perform a quick format to format the volume with NTFS.
Step 4. When the wizard is done formatting the new volume, you can assign or rearrange drive letters as needed. For example, changing drive letters may also put your CD/DVD drive in a more logical order.
To do that, right-click a volume or the CD/DVD drive and choose Change Drive Letters and Paths. If a volume hasn't got a drive letter yet, click Add. Otherwise, select the drive icon in the dialog box and click Change.
If you're rearranging the letters on existing drives, you may need to change the drives in a particular order. Or you may need to give a drive a temporary letter (such as Z) to free up a letter for another drive; you would change the Z drive to something else later. Make your selection and click OK. Repeat for other partitions or drives until you have the order you want.
Step 5. When you're done with your partitioning chores, exit Disk Management. Insert your XP disc into the drive and restart your system, booting from that disk.
Step 6. Follow the steps to install XP. When asked for the target drive, select your new partition and press Enter. Because you already formatted this partition with NTFS, you can skip the formatting step. At the appropriate screen, arrow down to Leave the current file system intact (no changes) and press Enter. Continue the installation process until it's finished and XP has started.
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.
How to customize your boot menu
When you start your dual-boot system, the menu will appear for a few seconds. If you don't press any keys, eventually Windows Vista will start. Fortunately, you can change this if you don't want Vista to be your default operating system. You can also customize the waiting period before the default kicks in.
Here's what to do:
Step 1. Click Start. Type systempropertiesadvanced and press Enter. Click Continue, if prompted by User Account Control.
Step 2. Under Startup and Recovery, click Settings.
Step 3. At the top of the dialog box, select the operating system you want to start by default.
Step 4. In the box to the right of Time to display list of operating systems, specify the number of seconds for the options to stay on screen. Click OK.
Microsoft provides documentation of Vista's bcdedit command and its parameters in an article in the Windows Vista Technical Library
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