First you should enter bios and switch to CD boot priority to first. Then it will boot from CD otherwise you may be prompted after POST to press any key to boot from CD.

XP has a 64bit version for larger memory addressing, but most are 32 bit. If there's no data you want on it currently, then just insert the XP disc and choose the delete the existing HDD partition with Windows8 on it and follow the prompts and create a new partition, format and install.

XP is no longer supported so you'll also want to check if you can get Service Packs 2 and 3. I would also recommend checking the existing device support and grabbing all the "other" drivers you need such as network interface etc.