I don't think that the dos or Win 98 version of fdisk will see the Linux partition.
The key is, what you have now. If a Linux bootloader is installed and dual booting XP and Linux, don't delete the linux partition, or you will lose the ability to boot XP. If you do, than the grub reference on the linux partition will be gone and booting XP not possible.
You noted that XP was pre-installed. Does this mean that you do not have a full copy of XP which will allow you to restore the Windows bootloader from the cdrom disk?
Some have reported that they have been able to recover the XP bootloader with a Win 98 boot disk, others have failed. This depends on a number of factors.
If you don't need the disk space and you can still boot into Windows, I would leave well enough alone.
If as JustBill noted, Windows was "taken over" by Linux (if your son allowed a full takeover of the hard drive), then you have no XP anymore.
In any case, you need to do two things;
1. Restore the Windows bootloader or uninstall the current linux bootloader (grub or lilo, whichever you are using).
2. Reclaim the disk space, if you need to, with a partitioning manager.
Let us know what your situation is and we can advise further.