You should be able to download and burn to cd something like knoppix. Or, if you have some other distro on install cd, boot from the cd.
Then, edit the running version of /etc/fstab to list the linux partition and a mount point for it. If the mount point doesn't exist, create one: something like 'mkdir /mnt/linux' would work.
The line to add to fstab would read something like:
/dev/hdb /mnt/linux ext2 defaults.
If you installed linux with some other filesystem type such as ext3, reiserfs, etc., use that instead of ext2.
Now the working fstab lists the mount point for the linux partition on hdb.
Open a term window and give the command: mount /mnt/linux.
That will mount the partition. Now, navigate to the /mnt/linux/boot/ directory and edit /grub/grub.conf (or /grub/menu.lst if that's what you're using), or find the lilo.conf file if you're using lilo as bootloader. Edit the kernel line to point to:
kernel (hd1,0)/vmlinuz root=/dev/hdb1
This assumes that you have only one partition on hdb, and that's where linux is.
Then, install the bootloader to the MBR. That should make linux bootable, but XP not bootable. Edit the bootloader conf file to add XP. There are tons of posts on this topic on these boards.
Once, these steps are complete, both OS's should be bootable from the linux boot loader.