problem in installing cent os
 

I have 160gb laptop. i installed vista in c primary partition which is 25gb and installed ubuntu in d primary partition which is 20gb. A remainig for my data.
Now i tried to install CENT OS by formatting ubuntu. I inserted CENT OS DVD and restarted and i selected to delete my /dev/sda2 which is showing 20480mb and it shown me free space.
but i tried to add partion /boot of 100mb it got added.
but, when i am trying to add / of 3000mb in the remaining 20380mb free space it showing an error message that no free space is available.

i am new to linux please help me out how to install CENT OS and format ubuntu.

there is a option in centos " remove all linux partition and create a default layout " choose this option and see if it installs

When you see the option :
"Remove linux partitions on selected .... "
then please click the message and choose :

"Create custom layout"
.. which will bring you to a window, where you
can choose your preferred partition.

i did custom layout and after that i made a partion /boot of 100 mb
and when i tried to make another partition / of 3000mb its gettin an error message like
cannot make partition enough space is not available..
please give me step by step procedure for installing

please.. i would be thankful to u...frenfrancis and Guru..

Well, you don't have to make space if you already prepared
a partition beforehand : just highlight it, and : "Edit" >
tick for 'Format' (ext3) and set the mount point ( / ).


Making the partitions ready before you use the CentOS
install disk is the preferred metod :
The "Gparted Live CD" is recommended.
..