You can have Ubuntu replace Windows, but that will reformat the partition and you will lose all data. Alternatively, you can install Ubuntu in another partition and have it installed side-by-side Windows. This will preserve the partition with Windows and you should not lose any data as long as you have enough free space to safely create the Ubuntu partition.
With the latter setup, you can mount the Windows partition in Ubuntu and access all your Windows 7 files from within Ubuntu. Programs that you run on Windows 7 will NOT work in Ubuntu.