It is easy to install xp on vista system.
Method 1:
1- Boot from your your xp cd and install it on any of your drive(except vista drive).
2- After installing Xp you will not found your vista boot option on the boot menu.
3- boot again from Vista DVD and choose the option of repair.
4- It will check an existing istallation of vista. And repair it quickly.
5- Now remove your DVD and boot normally.Hare now you will again find your visat boot option.
Method 2:
If you not have Vista DVD:
1. Install the previous version of Windows.
2. Log on to the older operating system and restore the latest boot manager by running the following command from the run dialogue box (fixntfs.exe will be in the \boot directory of the active partition which is usually C:\ Drive).
C:\boot\fixntfs /lh
3. Create a BCD (Boot Configuration Data) entry for the older operating system by specifying the following.
bcdedit.exe is located in the \Windows\System32 directory of the Windows Vista partition. To access bcdedit.exe from within Windows XP you need to use the Command Prompt (DOS Window) and point to bcdedit.exe.
Description is the description of the new entry for the older operating system.
bcdedit /create {legacy} /d “Description”
bcdedit /set {legacy} device boot
bcdedit /set {legacy} path \ntldr
bcdedit /displayorder {legacy} /addlast
4. Restart the computer in order for the changes to take effect.