Ubuntu 10.04 is available from the Ubuntu homepage in various flavors. You can chose the one best suited to your need but the problem here is that most of them are ISO files and you will go around looking for a CD to burn the ISO.
Let me tell you, that is not necessary. You can install Ubuntu from a portable hard-drive or a thumb drive also. All you need is a simple application named Unetbootin which is available for Windows and is built into Ubuntu.
Go to the Unetbootin SourceForge hosting page to download it. The download is sized at under 500 KB and has options to create a bootable drive for installing Ubuntu from a downloaded ISO. It can also download Ubuntu and create the bootable disk automatically.
Go to the Create Startup Disk option under System->Administration->Create a USB startup disk. Here you will find a simple interface explaining it all.
These are the two methods to create bootable Ubuntu disks in both Windows and Ubuntu. Now that you know how to install ,get hold of a thumb drive, download the ISO and follow this installation guide.