How do I backup the pre-installed Windows before installing Ubuntu?

I want to install only Ubuntu on my new laptop. But before that, I would like to make a backup of the Windows OEM installation on my laptop and then later, in case I wanted to sell the laptop, I would copy it back no the disk to increase my chances of selling it for a reasonable price.

I want to do this without having to start Windows at all (i.e. accept EULA, etc.), and no, I don't want to dual-boot.

You should use a bootable tool like Clonezilla to easily back up the OEM install (as well as any recovery partitions) on the hard disk. Clonezilla understands NTFS, so it will only backup (and optionally compress) the actual amount of data.

Using a raw block-level tool such as dd, while possible, is not recommended because it will probably use much more space than a smart tool such as Clonezilla.