Yep, low privileges only isolate viruses. A virus running as superuser can infect the entire system. A virus running as a low-privilege user can only infect what the user has access to. And all this applies equally to Windows as to Linux.

>(b) How valuable do you think it would be to a virus writer to infect Mastercard's data center? It isn't riddled with viruses.

Have you heard of Stuxnet and how it infected nuclear reactors?