Normally you would do it the other way, older OS first then the newer OS. I'm thinking when you installed XP it wiped out the Windows 8 Boot loader and replaced it with its own. To get 8 working again you'll likely have to do a boot repair. Do you have the Windows 8 install media?
I wrote windows 8 installation file to my USB
I have had this happen on my Windows 7/8 system. Usually I just plug in my installation usb with Windows 8 on it, then use repair windows and it finds the boot entries for each and then puts the dual boot back into play as normal.
It should be the same for XP, but no guarantees.