As per HN guidelines, "please submit the original source". The posted article's source is this article from The Guardian:

http://www.theguardian.com/technology/2016/jan/16/trident-ol...

"Microsoft ended security updates for Windows XP in 2014, meaning that the Vanguard submarines could be vulnerable to viruses, malware, and cyberattacks."

I'm pretty sure that the systems that are running these subs aren't being used to surf the web. They're probably not even networked with any systems outside the navy, so it's not likely that they'd be exposed to malware. I'd also guess that mission-critical systems are not physically exposed in such a way that a random sailor could plug a USB stick into them - the CPUs are probably inside locked compartments with highly restricted access.