This is an answer I wrote for someone else but it will help you for certain so I am copying it here.
It would be easiest to just start from the beginning so lets do that, okay?
First, boot to your USB or DVD and let Ubuntu start a live session. You will see an icon on the desktop that says Install Ubuntu on your desktop. Launch (click on) it.
When you get to the screen where it asks you if you want to replace an existing install or install next to it or Something Else, choose Something Else.
Now you will see a screen that will let you see the hard drive you want to install to. Make sure you choose the correct drive you want to install ubuntu on! On that screen you will need to first delete any existing partitions so you can start fresh.
Start at the bottom of the listed partitions and right click on each one and choose Delete. After you have done this to all the partitions, click on Apply.
When you have deleted everything your drive will show as completely unallocated space! Sometimes some partitions cannot be deleted right away so delete all you can at first and click Apply, then start over from the bottom until they are all deleted.
Now you want to create the necessary partitions for you to install Ubuntu.
Create the following partitions:
a boot partition
/boot and give it 250 MB of space, choose
ext4 for the format.
a root partition
/root and give it 15000 MB of space and
a swap partition
/swap and give it 4096 MB of space (it will format to swap)
a home partition
/home and give it the rest of the space and format it to
Now just finish the install and it will run beautifully.