I have windows 2000 on my desktop and i have got free space of 20 GB.
Do you mean you have 20 GB free in the partition where Windows is installed, or you have empty space that Windows cannot use? If it's the former (as in you can see the free space when you view the C drive's properties), you'll have to shrink the existing partition to make room for Linux. If this is your situation, DO NOT install Fedora until you're sure that the partition has been properly shrunk. I can explain to you how it's done, so ask if this is what you need to do.
I want to install fedoracore5 on the free space without deleting existing Windows 2000 Pro OS.
Next, you're going to need to burn them to a disk. Be careful, because simply burning the .iso file to the disk isn't going to work. These are images of CDs, so you're going to need a special utility. ISO Recorder (free, donations accepted) is one such tool, although there are tons out there.
The rest is fairly simple - boot from the CD, go through the installation steps, make sure that Fedora installs itself in the free space and doesn't delete Windows, and configure GRUB, which will allow you to dual-boot Linux and Windows. I won't go into detail here, but this is a helpful link which can guide you through the installation process: http://www.informit.com/articles/article.asp?p=605033&rl=1
Have you thought of installing Fedora on a virtual machine on Windows using something like Vmware or MS Virtual PC?
Using emulation tools to install an operating system can be both educational and useful, allowing you to work in an environment which you don't need to worry about messing up. VMs create virtual hardware, eliminating the need for drivers and other such things that Linux is prone to. It's also convenient in the sense that you don't have to reboot to switch operating systems.
However, installing Linux has many advantages. For one thing, it's way faster to run it natively rather than emulating. You also don't get the "true" Linux experience by simply runnning it in a virtual machine; something about full screen makes it a hundred times more fun. :)