@jk Gotta go old school for that sort of longevity
1000 years sounds possible
Depending on your definition of computer it's already been done with the Antikythera mechanism
https://en.wikipedia.org/wiki/Antikythera_mechanism
Put that in a vault and not at the bottom of the sea and I bet it'd still work
More useful would probably be a Babbage Analytical Engine built with modern materials