Hello ServerIP, welcome to NST.
Get
EasyBCD 2.0, add an XP entry (Type: "../NT/XP"), and let it auto-configure. Then you'll have a multiboot.
Vista must have been "system" (meaning it contained the boot files of both systems) before you formatted it, so once you did, you lost the XP boot, because you lost its boot files. However, auto-configure with EasybCD, and you'll be booting again in no time.
Cheers.
Jake