I have downloaded Ubuntu 12.04, made both a bootable usb stick and a iso image dvd. I am not able to install it on my laptop running on windows or my desktop running only on ubuntu 10.04. Further, my desktop has now neither the old version or the new one and I am stuck. Unfortunately ubuntu no longer ships free CDs and we are stuck with the downloaded version which may be missing something. After entering user name and password, there is some error reported on both laptop and desktop after which it does not respond at all. Kindly help.
A good idea might be to download the alternate installation package, if you are sure you want to install, and not just try it out.
When you download that ISO image, there is a way to check the ISO before burning, using the md5sum. Once you know the ISO is valid, you can check the burned CD to make sure it is also correct.
For the official Ubuntu instructions to install, and to troubleshoot installation problems, you can read this guide. There is a section on troubleshooting that explains how to check the CDs, etc.