If I understand correctly - You've Got a 120GB primary disk and a 15GB slave. XP is installed on the Primary and Linux is on the slave? I'm assuming that you let Linux load Lilo or Grub as the bootloader is this true?
If that's the case then fdisk /mbr would simply replace the MBR on the primary with a generic entry. Looks like a pointer to LILO/GRUB is in there now. I Don't think XP tries to be "cute" with the placement of the partition table so it would leave your data intact. I've used it in the past to remove LILO without issue.
If it turns out that you can't boot just copy: BOOT.INI, NTLDR, and NTDETECT.COM to a floppy prior to running fdisk. You can boot it off the floppy and repair then.
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CLS