I am installing rtlinux on PCQLinux 2008.i have done make config,make dep,make modules,make modules_install and make install.My grub.config file is:

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/VolGroup00/LogVol00
# initrd /initrd-version.img
#boot=/dev/hda
default=1
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title PCQLinux (2.4.21-rtl3.2-pre3)
root (hd0,0)
kernel /vmlinuz-2.4.21-rtl3.2-pre3 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-2.4.21-rtl3.2-pre3.img

title PCQLinux (2.6.32.4)
root (hd0,0)
kernel /vmlinuz-2.6.32.4 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-2.6.32.4.img

title PCQLinux 2008 (2.6.18-53.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-53.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-2.6.18-53.el5.img

when booting with PCQLinux (2.4.21-rtl3.2-pre3)boot image i get foolowing error:

Kernel panic : VFS : Unable to mount root fs on 00:00

Please help me...

That error usually indicates a bad or nonexistent initrd file. Check to see that the initrd file is really there, in /boot. If it is, or if it isn't, I would generate another one. What I can't figure out is why you want to use a 2.4 kernel.

initrd file exists in /boot.to make initrd image i used command
# /sbin/mkinitrd 2.4.21-rtl3.2-pre3-new.img 2.4.21-rtl3.2-pre3
but it gives error:
No module uhci-hcd found for kernel 2.4.21-rtl3.2-pre3, aborting.
rtlinux 3.2-rc1 having only patches for 2.4.xx kernels only so i used that.