[TeMPOraL]

You could if someone made one, presumably. OS is just a program, after all - and 20th century hardware couldn't stop you from completely rewriting the software on it.

The problem today is that modern computers are designed to prevent this, by means that can provide mathematical proofs you won't be able to defeat the protection in any useful sense before the Sun burns out. You have tamper-proof fuses embedded in microchips, and some systems have cryptographic hashes in every major component to prevent you from replacing something too hard to reprogram, etc.

We're yet to see a fully locked down computer (smartphones are close), but the tech for it is there.