Hi,
BE CAREFUL ! There are a lot of risks you have to know , if you want to install WinXP secondary on a working VISTA machine. Without exact knowledge of the present partition scheme, there is no advice possible ! Think of recovery partitions, hidden/ diagnostic programs or "closed-lid" operation systems for media, wlan or so on a laptop.
Generally you should use VISTA's own partition manager to shrink its partition from the end, thus gaining the space for the second system (Win XP) behind, which should also be partitioned and formatted by VISTA, even if it gets reformatted later. It's not left to you, how much space you get: VISTA "grants" it, because it does not move its system data, which is spread over its partition right from its installation , and the first found block from the rear is the stop point. DO NOT FORGET TO BOOT VISTA AFTER THESE ACTIONS, so that it is able to take notice of the changes and "settles" its filesystem, BEFORE YOU CONTINUE !
It is strongly adviseable to backup the MBR and all PBRs. Whenever possible keep the partitioning scheme between the MBR and the VISTA PBR untouched ! You should know, that there are programs on the VISTA installation CD/DVD, which can restore a corrupted boot configuration, but its always better, you don't need them. For the installation of WinXP, you have to have a primary (bootable) partition, but it is possible to move it in a certain stage of the process on another (yet extended !) partition. "NTLDR","boot.ini" and "ntdetect.com" are in an early stage copied BY YOU to the (bootable) root folder of the VISTA partition and automatically integrated by its "bootmgr" (syntax of "boot.ini" must be familiar / lines adapted!). If there are already three primary partitions used by the manufacturers "shipped" installation, and you intend to install logical drives in an extended partition for XP, you have to temporarily install XP on a second HDD or a big USB-Stick (yes, that works with some precautions !), then to interrupt the process at a given point (you can set a parameter for that) and to copy all files manually to the planned final location (as does every OEM). There you resume the installation by booting via VISTA. If there is a spare free primary partition, you should "disguise" the Vista partition as a non bootable Linux partition by the filesystem ID in MBR, activate the new XP partition and start the installation, stopping at the same point as above. Then "reanimate" and reactivate VISTA, copy XPs boot files(s.above) to the VISTA root folder and continue by booting. NOTE, that you ALWAYS have to change "boot.ini"-lines before !
If you want to create further logical drives in an extended partition with its working XP system, feel free to use "GParted" for that purpose, but better do NOT move WinXP any more.
Bye
cmdr