I have absolutely no experience with Linux, and I desperately need to get my computer back up and running again with Windows.
How do I remove Ubuntu and reinstall Windows?
Editor's note: many of the answers are about removing Ubuntu from dual-boot but keeping Windows (which is a bit complicated), while other answers are about removing Ubuntu from single-boot (which is easy: basically just format the disk while installing Windows). The question as written is ambiguous between dual-boot or single-boot.