Deployment Solution

what are pre defined computers  in OS deployment. Can somebody explain me the concept. When I boot the machine to pre boot why does he hostname change....???

Predefined computers allow you to assign deployment jobs to machines that the NS doesn't actually know about yet.  And in WinPE, hostnames are randomly generated to prevent any name collisions.  

To Clarify,

You are able to setup a computer in advance of it being built or adding an the Altiris agent to a machine.
This is done using Mac Address and serial number along with other required details like Name.

We have used this previously when we have bought large orders of varying devices and need to allocate them to different business sectors, builds etc.

e.g. predefining the names and build so that when they first boot and go in to PXE they already know what they need to do to be built and added on to the network. you can do all the work ahead of time and allocate the names of required jobs etc. this also helps ensure that the correct models / speced machines get built for the right purpose.
I could preallocate these machine names: WS-SITE1-SALES1, WS-SITE2-SALES13, WS-SITE2-WHS12 and they would just need to be plugged in and would already know what they are going to be without any user intervention. knowing to build 1 sales machine for site 1, 2 machines for site 2 one being sales and one being warehouse based off the serial / mac details you have advised and the name you've specified.

Default host name for preboot would be serial number of that machine

You can also assign specific hostname to particular machine by updating predefined computer's page ( Settings -> Deployment -> Predefined Computers )

Once you boot the machine into preboot, next time onwards this machine will get same hostname every time.