Thanks for your reply.

Unfortunately this confuses me

I do not have an infected computer. I assume that any computer which has ever been connected to the internet is potentially infected.

So in your proposed solution

a) I still don't know how to get a clean ubuntu (because I would have to download it - so again internet - again pot. infection)
b) I would connect my 'clean' computer to the internet - even if it is through tor - it still has a connection to the internet and therefore security is potentially breached.

Unless I don't get it...