Step 1 – Download the Ubuntu 10.04 Desktop ISO image, as per your your hardware architecture (i386 or amd64), from here
Step 2 – Burn the ISO image .
Step 3 – Insert or leave the CD in your CD/DVD-ROM device and reboot your computer. Hit enter when the boot screen appears and wait for it to load into memory.
Select your location
Select your keyboard layout
On the third screen, you will be asked to select the keyboard layout that suits you best (default is U.S. English). You can also test your keyboard on the little text input field situated at the bottom of the window. Click the Forward button when you have finished with the keyboard configuration.

Hard disk partitioning
The hard disk partitioning is an easy task, Please select the option very careful when ever you have more than one operating system.
Who are you?
Here you must do exactly what the title of this step tells you to do. You must fill in some fields with your real name, the name you want to use to log in on your Ubuntu OS and the name of the computer (automatically generated). Once you have finished with this step, click the Forward button again (for the last time).

Finally Ready To Install
If you have successfully arrived at this point, then you are definitely ready for your new Ubuntu operating system. So, what are you waiting for? Click the Install button NOW!
Step 5 – After approximately 6 minutes, take the CD out and Restart the system for access the new ubuntu OS.

Ubuntu 10.04 Login Screen
Click here For Trouble Shooting the ubuntu Installation Issue