I have Linux Advance Server 3.0.Earlier I had linux kernel version 2.4.21-4.ELsmp than I upgraded it to 2.4.21-9.0.1.ELsmp.I have one doubt that can I boot to rescue mode with the earlier installation cd which is having linux kernel 2.4.21-4.ELsmp.Can I make changes booting to that installation cd with rescue mode if there is any corruption in any file.
Thanx

You can, but there is no need to, the installation media in rescue mode and work just fine. You can create you own bootable rescue CD if to want also;

http://sourceforge.net/projects/rescuecd/

Differences in kernel versions will not cause any problems in accessing rescue mode. However, if you experience filesystem corruption on your root partition you probably won't be able to mount it under /mnt/sysimage

This means that i can't repair fstab file through old installation disk
Thanx

Not necessarily.

As long as your filesystem is not corrupted (partition tables are okay) you will still be able to edit files on your system using an old installation media.

Okay I will try to edit the files.If the file system is corrupted than you can not even be able to boot from rescue disk (Installation media of same kernel version).

Almost true, with the rescue boot you still can repair the filesystem(s). FYI: you always want the filesystem(s) to be either not-mounted or read only to do repairs.

See; man fsck

See if this explanation helps ...

A "rescue CD" (or "liveCD") is simply a bootable Linux system on a CD or DVD: one that does not require any hard drive in order to run. You just put the CD into the drive and reboot the system from that CD.

Now, you'll find yourself in a completely stand-alone environment in which any of your hard-drives can be mounted, repaired, inspected, optimized, or what-have-you without any of those drives being "the root filesystem." Furthermore, since the Linux that you are now running is fixed onto the CD, you can't affect it by whatever you do.

So, for example, if you find that you flubbed a change to the Grub or LILO configuration-files and can't boot your machine, you can boot the CD, mount the hard-drive, cd to the proper directory, and edit the file. Another example is doing filesystem verification and repair: you can't do that with a mounted filesystem, but if you have booted from a LiveCD, the hard drive filesystem(s) can be mounted or dismounted at your discretion. Therefore you can perform these tasks.

Finally... a few distros still mumble about "rescue floppy disks." Fuhgeddaboudit! Every computer you'll find these days can boot from a CD or DVD, and more and more of them don't even have (nor do they need) a "floppy" drive.