Before you begin, you want to make sure that all of the data from your Windows installation is safely backed up, or moved to your Ubuntu partition. Also read these instructions briefly to ensure that they make sense.
- Boot from the Ubuntu live CD.
- Open the GParted program. Your hard disk is split up into 'partitions'. You will have three (3) partitions: One for Windows, one for Ubuntu, and one (the smallest) swap partition.
- Once GParted opens, identify the Windows partition. The partition type is NTFS and will be outlined in greenish blue. The Swap area will be red and the Ubuntu partition will be navy blue, but you want to keep these.
- Right click the greenish blue Windows partition and click "Delete partition".
- Now, right click the navy blue ext4 Ubuntu partition and click "Resize/Move Partition". A dialog box will open - drag the right or left edges of the navy blue box so it fills up the maximum amount of space possible.
- Click OK to close that dialog box.
- Click "Apply" in the main GParted window to apply these operations. Confirm that you want to, if you are 100% sure that you do not need Windows any more. The operation will begin, and it might take a long time - but be patient while it finishes.
- One finished, reboot into your Ubuntu installation.
- Open the Terminal with ctrl+alt+T and type
sudo update-grub, then Enter. Type in your password, which will not appear, and press Enter again. This will remove Windows from the Grub menu.
Then, you are finished.
But seriously, make sure you don't need anything (internet history, etc.) from your Windows installation! I find Firefox Sync, Microsoft Skydrive, and Google Drive very handy for ensuring a smooth migration to a new OS.