If it's not games you're playing, it's often better to install a Virtual Machine program such as VMware or VirtualBox and load your XP on that.
Another, even easier option is described here:http://lifehacker.com/5965889/how-to-run-windows-xp-for-free-in-windows-8
If you really need to run XP natively, make sure there are drivers for all your hardware first.
Best thing would be installing them on separate HDDs, i.e. unplugging your Win8 HDD until you're done installing XP. After that, you can use your motherboard's boot menu (often called by pressing F11 during POSTing) to choose which HDD to boot from.