I'm assuming this model comes equipped with dual hard disk drives as you are likely changing the boot priorities in the bios. You will need to install a bootloader onto the primary hard disk enabling you to choose between the os's without modifying thre bios. Do a google search for GRUB and read the associated wikis.
Another option you might consider, rather than using a multiboot configuration, is to use Windows 7 as your OS and install Virtualbox (free from Oracle). Then you can create a virtual machine that runs XP, or a multitude of other OS's
That would eliminate all of the bios configuration changes and give you access to needed OS's in a far smoother operation.