"my XP disc doesn't run on start up with Vista installed as the OS"
The intalled OS should not determine your system's boot order, you might want to go over your manual & see how to properly configure the boot order for your notebook.
Normally, the BIOS dictates what will be the primary boot device & you can change this to suit your incidental needs.
What should happen here in order to get your Vista /XP dual boot setup involves.....
- Setting the DVD drive as the first boot device.
- Booting from it & installing XP in a different partition (created prior from Vista using the *shrink* feature from the disk management console)
- Using a program like EasyBCD to edit your boot loader once all of the above is accomplished.
Good luck!