I am new in Linux. I had installed Redhat 9 on a old PC. Recently, I found that the root partition (/) was corrupted. The Grub gets into the grub> command prompt instead of the Linux. How can I re-installed the root partition. The /usr, /etc, /home, /temp, /var are in separate partition each. But the /boot is in root partition. Any advice?

Something might just be messed up with your grub config file or maybe even the mbr. does it give you any errors or does it just show the grub screen?

First thing I would try is to boot with a Knoppix CD and see what's going on. Is the partition really corrupted, or just a simple grub misconfiguration as donguai said? You can run fsck's from Knoppix to check for (and hopefully fix) filesystem problems. I wouldn't jump right to "How do I reinstall the root partition?" just yet. A little more troubleshooting (or more information presented here when asking for help) might save you some work. Of course if root is really corrupted badly, you just may have to restore it. And you should try to figure out why it's corrupted if that's really the case. You said "old PC". Could you have a bad harddrive? Hopefully you have backups, but maybe not if you're new to Linux and haven't gotten around to this important step yet.