I want to install CentOS upon Windows 8(dual boot) replacing Ubuntu 11.04. I have two ISO files (dvd1 and dvd2). Suggest me ways to install the OS from USB. I tried using Unetbootin but it is not booting. Do I need to merge the iso files before creating the boot usb?
If you have Ubuntu working still, you can transfer a bootable image to your USB with something like this:
dd if=CentOS-6.6-x86_64-bin-DVD1.iso of=/dev/sdb
Substitute /dev/sdb with the device for your USB drive.
Then, reboot with your USB and go through the installer. Once you are done installing, you can install additional packages from DVD2 from within CentOS.
See InstallFromUSBkey for more.
You don't need to merge the two DVDs to get reach your goal of getting CentOS installed.