From W8 Disk Management check the flags
Disk Management flags have the following meanings
"boot" = "this is the system you're running"
"system" = "this is where I found the boot files for the currently running system"
"active" (on the first HDD in the BIOS boot sequence) = "this is where I started the search for the boot files"
"active" (on subsequent HDDs in the BIOS boot sequence) ="this is where I will look if I don't find something in the MBR on the first HDD"
If W8 put its boot files on the XP partition (XP marked "system") then you can format the W8 partition and its bootmgr will continue to operate from where it is.
If W8 put its boot files on a separate small partition, you can keep that and format the OS.
If you want to get rid of everything that was W8, and that includes what is now "system", you can use EasyBCD > BCD Backup/Repair > Change boot drive while W8 still exists (point it at XP)
Changing the Boot Partition - EasyBCD - NeoSmart Technologies Wiki
and once XP is "system" and booting successfully, you can remove W8.