I have already installed Windows 7 on C: drive. Now I want to install Ubuntu on E: drive which is a free partition. How do I proceed?
You can install Ubuntu on a separate drive by booting from a CD/DVD or bootable USB, and when you get to the installation type screen choose something else.
The images are instructional. Your case might be different.
Be careful to make sure that you're installing on the right hard drive. If by mistake you install on the C Drive it can cause a problem with Windows.
Just make sure that you read the instructions, especially which hard drive to install to, since Linux lists drives different than Windows. Check the capacity of the drive that you want to assign to Ubuntu, and make sure that you have select the correct hard drive.
I suspect you are using Wubi, which I have never used so this could be completely wrong but you should be able to burn a live CD and boot from it, this might let you select the other disk to install to when choosing the install location.
You can also mount different parts of the OS to different drives. I have my / on my 120gb SSD and my /home on my 1tb HDD, this will place all the user home folders there and most apps there (I use up a lot of space and needed to do this). You can mount seperataly post-install but it is much easier during installation