first of all i installed windows me in (hd0,0) and red hat linux ES 3.0 in (hdo,1)
partitions. and note down the contents of grub,conf file. whcih are give here

default =0
timeout=10
splashimage=(hdo,1) /boot/grub/splash.xpm.gz
title red hat enterprises linux ws (2.4.21-4.EL)
root(hdo,1)
kernel /boot/vmlinuz-2.4.21-4.EL ro root=LABEL=/ hda-ide-scsi
initrd /boot/initrd-2.4.21-4.EL.img
title window me
rootnoverify (hd0,0)
chainloader +1




now i installed fedora core 3 on free partition(hd0,5)
after installation i just enter the entris of red hat linux in grub.conf file under fedora

core 3 distros.

now but when i choose red hat to boot it shows meesage
kernel /boot/vmlinuz-2.4.21-4.EL ro root=LABEL=/ hda-ide-scsi
error 15 file not found
press any key to continue

and after pressing key it return back to boolloader menu.
rest of two os running very well.

tell me the reason

It seems to me you made a typing error. Instead of root(hdo,1) it should be root(hd0,1) and instead of splashimage=(hdo,1) /boot/grub/splash.xpm.gz it should be splashimage=(hd0,1) /boot/grub/splash.xpm.gz.