You need root privledges to edit system files.
Either log into to Xwindows as root or open a command prompt and type "su" then your root password once your prompted for it. And I don't think you want to edit the boot file. What distro are you running? your bootloader is either lilo or grub.
If it's lilo edit /etc/lilo.conf then when your done run the command "lilo" if it's grub edit /boot/grub.conf If you need more help feel free to post.
Moved to our linux forum.