Dear Friend,

When i am installing linux on the pc i had already install the win98 on the pc and i like to install the linux server installation while i choose the automatic partation future it will give the erro that

You have not definedd a root partation (/), which is required for installation of red hat linux to continue

So provide me the detail how can i do that

It's installation process issue, so much depends on the distro you use.
First, I'd not choose automatic partitioning. Make it manually, it will be better.
Your problem may be that Windows has the whole hd and Linux installer can't find free space to install. In that case, you need a program capable to resize the Windows partition and free some space. You can use Partition Magic, fips (coming with RedHat), parted etc. Mandrake installer can resize a partition during installation. When it comes to partitioning, click on the Windows one, click "resize" and decide how much space should be fried.
You'll need at least 2 partitions for Linux: root partition (/) and swap partition. You can also create one for users home directories (/home), etc.