If you have installed Linux after XP installation and if you have chosen the boot loader to be written in MBR, this problem may arise.
If you have chosen lilo as boot loader follow this: [You can verify this by seeing the lilo prompt while booting]
Open the /etc/lilo.conf
Add the following entries
other=/dev/hd[x][y]
label=xp
where x can be a,b,c,d and y is partition number.
For example, If u hv installed XP on Primary Master disk and on partition 2, give it as
other=/dev/hda2
close /etc/lilo.conf
and then run
lilo
It should update the boot record.
After rebooting you will be able to see xp also in the boot menu.
Regards,
VT