This is useful for, eg, installing debian over the network with no cd drive or usb stick.

Download the appropriate initrd.gz and linux kernels from whatever netboot intallation location (two seperate files), and put them somewhere like /boot/install/. Then edit /boot/grub/menu.lst and add an entry like:

title   new install
root    (hd0,0)
kernel  /boot/install/linux ramdisk_size=128000 root=/dev/rd/0 rw
initrd  /boot/install/initrd.gz

Edit the ramdisk size, ramdisk device path, etc as needed.