i have a iso file downloaded frm linuxiso.org. i tried harddisk installation of this file. i used RedHat bootdisk for it. Did i make any mistake? How can i do it successfully?

You can not install from an ISO image. The image must be burned on a CD and then booted from the CD. You can not just copy an ISO to a CD there are special procedures for writing a disk image to disk. if you are using linux see the instructions for using cdrecord to write ISO images

i read instructions somewhere that u can install it from a harddisk, the only thing u need is a bood disk with boot.img on it. so i just did the same.
ya another fact, i did the same thing with redhat package, i had CDs of the packeage but instead installing it frm cd i took the image of those CDs and put in a Extended partition. then with the use of redhat bootdisk, i installed it frm my harddisk successfully.
i dont understand why it didnt work in the former case

If you are installing from a partition on a hard disk to another partition on that or another hard disk you should have downloaded the distribution files not the ISO file; an ISO file is not the same as copying the files from as cd to your hard drive, it is a special format to allow you to recreate the cd sector by sector. You then would be able to make the boot floppy (see the handbook at the FreeBSD home page) Please read the section on partitioning and have your hardware information written down. It is a little more hands on than linux but you will like the performance, even more so on older hardware with less memory. I still say burn the iso to a cd if you have a burner. But read the installation instructions all of them and you should have very little trouble.

Also you have to have the boot floppy for the program you are trying to install, if you are going to install from a hard disk. Detail is everything in a computer program read the Handbook until you understand the step by step of installation and partitioning, you need seperate disk partitions for each operating system. If you are planning to replace redhat with FreeBSd you need to have the disk or a good high speed internet connection.