Removing an Ubuntu installation

I first had windwos 7, then I installed Ubuntu alongside and when my laptop starts I can choose between ubuntu and windows (the ubuntu boot screen) .

Now I have installed another ubuntu inside the windows 7 using virtual box. I want to remove that first ubuntu installation and free up its partiotion. Can I do it without making any problems for the windows and boot screen?

You should try OS Uninstaller, that can be used from a live usb, burned into a CD or installed on your Ubuntu.

To install it on your Ubuntu OS:

Open the Terminal (Ctrl + Alt + T), then type:

sudo add-apt-repository ppa:yannubuntu/boot-repair

Press enter and then type:

sudo apt-get update; sudo apt-get install -y os-uninstaller && os-uninstaller

The OS-Uninstaller will be now available, so open the Dash Home and type OS-Uninstaller, then press enter to initialize it.

For information on how to use it access the official article at Ubuntu Documentation (https://help.ubuntu.com/community/OS-Uninstaller).