Microsoft stopped supporting this now quite ancient operating system back in 2014, leaving it vulnerable to attacks (although the company took the unusual step over the weekend of rushing out a patch to deal with the Wannacry threat). More recent Microsoft systems, including Windows 10, 8 and 7, can also be infected by the ransomware virus.
Windows XP is so old that Microsoft was no longer offering free software updates for it.
Microsoft had released a security patch in March, but computers and networks that hadn't updated their systems were at risk.
The ransomware attack was particularly malicious, because if just one person in an organization clicked on an infected attachment or bad link, all the computers in a network would be infected, said Vikram Thakur, technical director of Symantec Security Response. The only thing is that not numerous users always keep their versions up to date because you know how annoying the Windows updates are.
Britain's health system has recovered from the disruption, with 97 per cent of the country's health service trusts now "working as normal", Interior Minister Amber Rudd says.
Krishna Chinthapalli, a doctor at Britain's National Hospital for Neurology & Neurosurgery who wrote a paper on cybersecurity for the British Medical Journal, said many British hospitals still use Windows XP software, introduced in 2001.
What computers are affected by the cyberattack?
Europol, the European Union's police agency, said the onslaught was at "an unprecedented level and will require a complex global investigation to identify the culprits".
The National Cyber Security Centre (NCSC) is warning people to remain vigilant in the wake of Friday's ransomware attack, which affected thousands of computers around the world.
Two researchers in their 20's had halted the ransomeware attack on Saturday after discovering and activating the software's "kill switch".
Romania's Dacia Automobile Plant had to partially interrupt production on Saturday following a cyber attack that affected some of the information systems.
"This particular attack was relatively easy to shut down", said Bryce Boland, Asia Pacific chief technology officer for FireEye, a cybersecurity company.
It was too early to say who was behind the onslaught, which struck 100,000 organizations, and what their motivation was, aside from the obvious demand for money. Cyber criminals targeted users in 150 nations, including the U.S., Russia, Brazil, Spain, and India, along with major government agencies, such as the U.K.'s National Health Service and Germany's national railway.
Smith warned governments against stockpiling such vulnerabilities and said instead they should report them to manufacturers - not sell, store or exploit them, lest they fall into the wrong hands. Tell us what are the anti-virus programs that you use and other precautions that you take to stop these kinds of viruses.
"For those organizations who have not yet applied the security update, we suggest you immediately deploy Microsoft Security Bulletin MS17-010".