Hi
I have installed redhat linux9 and following is my partition distribution
/boot on hda6
/ on hda7
swap on hda8
I want to install Debian as second OS (dualboot with redhat). Will u please provide me detailed information that how can I do that. Do I have to make new / and swap partitions or can I use existing partitions. Please provide me detailed information as I have currupted my previous installation while trying to do that thing. Moreover please tell me that what changes I have to make in my GRUB boot loader.
Thanks.

Well, your question is a bit complicated to answer but for starters I can say that you can use the redhat swap partition also for debian. And you don't necessarily need anything else than a single / partition for debian. So, resize your redhat / partition to make space for debian. (I personally use the qtparted program that comes with systemrescuecd http://www.sysresccd.org/ to resize partitions.)

I've never installed grub to mbr because I use gag bootloader (which also comes with systemrescuecd). But since you've apparently already installed grub to mbr, you'll probably want to overwrite redhat's grub -- provided that debian installer appears to recognize your other partitions & OSs. If it doesn't, you can edit grub's menu.lst later on. If you're in luck, a new grub configuration that recognizes all your previous partitions is written during debian installation and you don't need to edit anything.

I suggest that you use the latest debian installer http://www.debian.org/devel/debian-installer/ . From the same page you'll find a detailed installation manual, which I suggest you read (and possibly print) before actually installing debian. Check out also the other documentation links on that page -- they'll answer most of your questions.