I've not tried it on XP but I have been in that situation many a times with Windows 7 and windows vista partitions. Win7 and vista recovery/boot disk you can launch command prompt. Always worked. Not sure if the commands are the same because they xp and vista have diff boot loader.

boot in to command prompt.

x:> diskpart
diskpart> listdisk ( this will list the disks you have )
select disk 0 ( zero is the first hard drive)
list part ( to know how many partitions you have)
select part 1 ( where 1 is the first partition where xp was, select 2 if xp was the second partition)
active ( this will make the partition active)

orrrr.... just to

x:> bootrec/fixmbr ( fixes master boot rec which is used to boot)