You can use Vista to create partitions.Go to disk management.
Use the link and create a partition of at least 20 gigs for XP to be installed on.
Windows Vista Help: Create a boot partition
You can ignore warning on link about installing Xp first you can still dual boot with Vista already installed.
Then the new partition will be assigned a drive letter then reboot=restart machine enter BIOS=Setup set first boot to CD/DVD rom put in XP disk save and exit.Choose the drive you wish to install XP on wich will not be C drive.You cannot down grade Vista=C drive to XP.That is why we create a partition or uninstall Vista.
Or do this to create a partition.
To create a new partition using the Vista CD proceed as follows:
1/ Switch on your PC
2/ Quickly insert the Vista DVD into your CD/DVD-ROM drive
3/ A message will appear asking you to 'Press any key to boot from CD/DVD
4/ Press any key
5/ A black screen will now appear with a progress bar along the bottom. The legend say's 'Windows is loading files'
6/ After the files have been loaded the Language screen will appear.
7/ Select your language from the list and press Next
8/ At the Installation screen press the Install Now button
9/ The product key window will now open
10/ Type your product key number into the available box and then press OK
11/ The End user Licence Agreement now appears. You 'must' accept this to continue
12/ The next screen to appear is 'Where do you want to install Windows'
13/ At the bottom of this window you will see an option marked Drive options (Advanced), click on this option
14/ In this Window select 'unallocated space' from the list of partitions and then click New
15/ A size box will now appear under the New option
16/ Type the size of the new partition into this box (or simply accept the default), then click the Apply button
The thanx button is a thankyou for help and a usefull post.