First format the sda2 partition with ntfs.
Reinstall Windows - select this partition.
Afterwards reinstall GRUB boot loader.
Boot from Ubuntu installation media.
Select Try Ubuntu without installing.
On Live desktop - open a terminal.
Execute the following commands :
sudo mount /dev/sda6 /mnt
sudo grub-install --boot-directory=/mnt/boot /dev/sda
sda = disk |
sda6 = system partition
When you want to keep Windows, it is recommended to keep the sda3 Lenovo partition as well.
It might contain important tools for recovering vendor specific - Windows related functionality.
In case there is no Windows entry in GRUB menu, boot Ubuntu ... open a terminal and execute :