Reposted to this answer your comment, since I'm not registered I can't repsond via comments.
How to install Ubuntu an existing partition, that has content on it:
- When presented with formatting choices screen choose "Something Else", and click continue
- Select the partition you want to install to. (eg. Sda6)
- Click "Change"
- Click the "Use As" menu, and scroll to find the file system you wish to use. (eg. Ext4)
- Check the "Format" box
- Choose a mount point (eg. /), and click okay
- Make sure boot loader is going to install to the correct drive. (eg. Sda)
- Click "Install Now", and proceed with installation setup
Please Note, this will erase all data on the selected partition. Alternatively you can choose not to format in which case directories and files that are on the partition will only be over-written if they use the same names as files and folders written by the ubuntu installer, though if you do this you must choose to use the filesystem as what-ever it is currently formatted as.
REPLY TO YOUR COMMENT:
Please Note, If you install the bootloader to your hard drive (eg sda), and you have Windows setup to boot from that drive, Ubuntu's bootloader (grub2) will overwrite the Windows bootloader. The Ubuntu bootloader will automatically add an entry for your Ubuntu install, as well as an entry for you Windows install, allowing you to boot both Ubuntu and Windows, however if you decide to ditch Ubuntu down the road, you will have to keep the Ubuntu boot loader (grub2) in order to boot Windows. Installing grub in this manner is standard, but not the only option.