I am Trying to install a 2.4 kernel in my 2.6 kernel machine.
I have down loaded kernel-2.4.22-1.2199.nptl.i686.rpm.
when i give my command rpm -ivh kernel-2.4.22-1.2199.nptl.i686.rpm
it says
warning: kernel-2.4.22-1.2199.nptl.i686.rpm: V3 DSA signature: NOKEY, key ID 4f2a6fd2
error: Failed dependencies:
kernel < 0:2.6 conflicts with udev-039-8.FC3.i386

What is the prob and how to solve?

Looks like you're trying to install an Aurox rpm which conflicts with an Fedora Core 3 package. Normally it's not a very good idea to use packages from different distros although in this case Aurox is based on FC...

The NOKEY error is due to the fact that you haven't installed the GPG key, but because you have udev installed and this needs a kernel 2.6 or higher you also get a dependency warning. Looks like you're not going to be able to install the 2.4 kernel without sacrificing udev.

p.s. including your distro in your profile will help giving better answers.

how to stop udev.
I am new this kernel operations please guid me

Never ever try installing a new kernel with rpm or any other pre packaged format, you will get errors most of the time.
Best way is to download the kernel source from www DOT kernel DOT org & then visit tldp.org and find out kernel compiling howto.
Anyways the documentation supplied with kernel is more than enough to help you out.Make sure that you read the dependencies reqd. by the kernel to compile.
Btw. it can take quite a while in compiling a kernel.

Take that opinion for what it's worth...
I've installed all my fc4 kernels through rpm and it works like a charm.
The fact that it forces you to have the right dependencies can sometimes sem inconvenient, but saves you from making stupid mistakes if you don't know exactly what you are doing. If you do know what's going on... help yourself and compile your own kernel(s)