I would add to that good advice, you need to know the partition of your /boot partition. check your /etc/fstab if you forget. if /boot is mentions translate it into grub speak.
If its not in your etc/fstab file then its a sub-folder of / and so you that.
the root command is to define where your /boot partition is or the sub-folder /boot is.
I am a little surprised you had no issues with FC2 and xp but have a peep at this if you like.