well there are 2 main options
get rid of that ms program.....hmmm but you spent money on it?
or having a cursory look at this,
you may elect to overwrite xp bootloader back into mbr and then mod it to chainload from xp to grub placed into the root partition.
pretending partitions were
use your technique to boot into linux (or have a live cd)
Do not proceed past this point unless you see that grub has found its files and you have a message on last line saying succeeded.
Naturally change that hd0,1 to something else depending on where you put the linux /boot partition or sub-folder to /
then grab a copy of the bootloader
dd if=/dev/sda2 of=/media/usb/linux.bin bs=512 count=1
change media to whatever you mount usb sticks to or floppy?
If you have ms partition mounted already in your fstab you can write it to that, eg
2) Then use xp cd to fixmbr to put xp bootloader into mbr
then follow this guide
so you are in xp after booting from xp bootloader and then you import off the usb stick etc the image of the linux bootloader.
You should use admin powers to modify the bootini file and you may need to change its attributes before doing so.
You may want to do some reading research before the attempt and know how to use a live cd to rescue grub if the xp way fails.