One fifth of websites on the public Internet are still running on Windows Server 2003 despite Microsoft having revoked support for the operating systems in July.
Some 175 million of the 870 million public hostnames were found by the Internet Services firm Netcraft to be using the ageing technology, which equates to some 600,000 web-facing computers.
In a statement, Netcraft said: "While Microsoft does not officially offer any support beyond the extended support period, reports suggest that some companies who have not migrated in time have arranged to pay millions of dollars for custom support deals."
Reports earlier this year indicated that Microsoft would charge companies $600 per server for a year’s extended support of Windows Server 2003.
Among the groups using the operating systems were NatWest, the bank ANZ and Grupo Bancolombia, with all three sites ranking within the top 10,000 most visited on the planet.
The Alibaba Group was also reportedly to be running 24,000 of the affected computers, some through subsidiaries HiChina and Aliyun, and China accounted for around a quarter of all computers affected, a similar amount to the US.
Despite the problems Microsoft was found to have slightly gained market share in webhosting computers between July and August to take 29% of the market, though Apache remains dominant with more than 46% of the market.
"Microsoft was responsible for much of the growth in web-facing computers this month, reversing the losses seen last month," Netcraft said.
"This month there was an increase of 15,668 web-facing computers powered by Microsoft web server software, accompanied by a gain of 6.1 million sites."
Microsoft’s abandoning of Windows Server 2003 follows a year after the company axed support for Windows XP, at the time provoking widespread concerns that consumers and businesses would be exposed to hackers.
At the time the British and Irish governments were among those to negotiate extended contracts for support, having not previously migrated to newer operating systems.