Thousands of websites are still hosted on Windows XP computers, despite the operating system reaching the end of its extended support period today. After today, Microsoft will stop providing automatic security updates for Windows XP, and Microsoft Security Essentials will also no longer be available for Windows XP.

Originally released in 2001, Windows XP is currently used by more than 6,000 websites in Netcraft's April 2014 Web Server Survey. Although China is often regarded as one of the most prolific users of Windows XP, only 3% of these sites are hosted there, suggesting that Windows XP has a predominantly desktop role in China. The largest share (nearly a third) of all Windows XP-powered websites are actually hosted in the United States.

Distribution of Windows XP-powered websites (logarithmic scale)

Notably, there are 14 US government websites still running on Windows XP, including a webmail system used by the State of Utah. Unsupported web-facing Windows XP servers are likely to become prime targets for hackers, particularly if any new Windows XP vulnerabilities are discovered, as no security updates will be available to fix them. To afford some breathing space, the UK Government recently struck a £5.5m deal for Microsoft to provide it with an extra year of support for Windows XP, although there are currently no Windows XP-powered websites under the gov.uk top-level domain.

One of the busiest sites still using Windows XP is TransFerry.com. This site was previously using Windows 2000, and perhaps more worrying is the significantly larger number of websites which still use Windows 2000. This version of Windows reached its extended support end date in July 2010, yet nearly half a million of today's websites are hosted on Windows 2000 servers, most of which are using the Microsoft IIS 5.0 web server software they were shipped with. This version of IIS is practically identical to that used by Windows XP (IIS 5.1).

Netcraft's April 2014 survey also found 50,000 websites which are hosted on even older Windows NT4 servers running Microsoft IIS 4.0, although three quarters of these sites are served from the same computer in Norway. One of the busiest sites still running on Windows NT4 is the Australian Postal Corporation's post.com.au, which has been using the same operating system for at least 13 years. Window NT4 and IIS 4.0 are also still used by Australia Post's Postbillpay bill payment service, airindia.co.in and by the French government's Ministère de l'Économie, des Finances et de l'Industrie.