Hi there.I need help with dual booting Ubuntu 10 and CentOS with Ubuntu installed first on my machine.

Thanx in advance....

Considering that CentOS is a Linux distro it should be available with Grub or LiLo so after you install it on another partition it may update-grub (or LiLo :P) and get you two (actually more but there's only two that will be the most used) entries in Grub (again, or LiLo) menu, one for Ubuntu and the other for CentOS.

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If CentOS behaves like Fedora (and it probably does), then when it creates grub it will make an entry for Windows (if found) but not for other versions of Linux. Ubuntu definitely sets up grub for everything it finds.

So, if you put Ubuntu on first there will be two options:

1. Let CentOS install grub to the MBR and then edit grub.conf to give an entry for Ubuntu, like this (using the original grub)
title Ubuntu
root (hd0,0)
kernel /boot/vmlinuz ro root=/dev/sda1
initrd /boot/initrd

2. Get CentOS to install grub on its own root partition and edit Ubuntu's configuration file (I can't remember what they call it) to call it, like thus (with grub2, assuming partition 3)
title CentOS
rootnoverify (hd0,3)
chainloader +1