Ubuntu Installation
 

My system is having centos and xp previously installed. I went for an ubuntu installation, but saw that the grub wasn't installed though the option was checked during the installation. What should be done to install ubuntu outside windows along with centos. Can anyone give suggestions?

This is what I get inside the boot folder of Ubuntu.. The installation was done but the grub isn't installed. (I just mounted the installed ubuntu partition and the boot folder is listed as below)
abi-2.6.28-11-generic System.map-2.6.28-11-generic
config-2.6.28-11-generic vmcoreinfo-2.6.28-11-generic
memtest86+.bin vmlinuz-2.6.28-11-generic

I am having a boot,swap and / partition for the Centos and went on for the Ubuntu installation with only a / partition of about 8GB.

If you have all on single drive, meaning physical drive, then you can boot into CentOS or and reinstall grub.
Else if you are using CentOS grub, you can manually edit the menu.lst or grub.conf file, whatever it is called in your distribution to add Ubuntu entry there.
I am assuming you can boot into CentOS but not Ubuntu.