To set the xp to boot by default you will need to edit the /etc/default/grub file as root user. I don't know what the editor used in elementary is but a standard Ubuntu uses gedit. In that case open a terminal and enter: sudo gedit /etc/default/grub
This opens the file with root/admin privileges so you can edit it. Replace 'gedit' above with whatever editor you use on Elementary and change this line:
Before doing this, go to the /boot/grub/grub.cfg file and count the menuentry lines to find which has xp and enter the correct number. Since in this instance Grub is counting from the number zero, if xp is the third entry, you would change the 0 to 2.
Some while ago I wiped the Linux partition from my hard drive but all this achieved was a Grub Error message on boot up. I reinstalled the Linux partition to enable me to get at the WP files.
Of course you would get that. Almost all of the Grub boot files are on the partition you deleted, only a tiny part is in the master boot record pointing to the other files on that deleted partition. The same would happen if you were able to boot with windows and you deleted the windows system.
If you are booting both xp and Linux using the Linux Grub bootloader and want just xp, the first thing you would have to do is install windows code to the master boot record from your installation medium. If you don't have the actual xp install CD/DVD you might have problems. You would need to check with microsoft or a windows forum on other options.