I want to install Ubuntu 18.04.1 replacing Windows 7 (installed on Drive:C).
Prepare and boot from a Ubuntu installation USB stick.
Choose Try Ubuntu
Open GParted software and make sure the internal drive (HDD) is selected, not the installation USB.
From left to right the first two (primary) partitions can be deleted. Those partitions are named as "System Reserved" and "WIN-7 (C:)" in your Windows Disk Manager. Please note they will show up differently in Ubuntu's GParted. Select > right-click > Delete each one.
Click Apply.
Now you should have ~100GB of unallocated space at the beginning of the drive to install Ubuntu and that plenty of space.
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