Microsoft is urging an estimated 70 million users of Windows 98 to upgrade as it ends support for the software.
From 11 July, Microsoft will no longer help users over the phone with any problems they have with the ageing operating system.
The firm will also stop providing security updates for Windows 98 from the same date.
Support for the software was originally due to end in 2003, but was extended following customer protests.
Shutting down
Products affected by closing down the support system include Windows 98, Windows 98 Second Edition, and Windows Me (Millennium Edition). Analyst firm IDC believes that more than 70 million users will be hit by the change.
The majority of these people are likely to be using the operating system at home, as by now most large businesses have phased out machines running the software. Many small firms are also believed to be users of the product.
Microsoft ended free support for this trio of products in late 2003 but continued to offer paid support and provide critical security updates. This too will now stop.
The change means that those still using Windows 98 and Me after 11 July could become more vulnerable to many security risks, as bug fixes for loopholes exploited by hackers will no longer become available.
In a statement Microsoft said it was "ending support for these products because they are outdated and these older operating systems can expose customers to security risks".
Mikko Hypponen from Finnish security firm F-Secure said that it was still providing updates for Windows 98 users who run its anti-virus package.
"In fact, 98 users are not at that great a risk as people might think: most of the new malware we see simply won't run there," he said.
"Nevertheless, if you want to be safe with 98: don't go online. Or upgrade to something that is supported with security patches," he added.
HAVE YOUR SAY
According to computer security firm Secure Science, at least one hi-tech crime gang based in Eastern Europe specialises in Windows 98 and produces viruses that prey on the software's weaknesses.
This is because users of it tend to have little knowledge of potential dangers - and are older, so are more likely to have substantial financial assets worth stealing.
Users of Windows 98 who have problems with the software may find an answer to their query themselves by hunting through the support information collected on the Microsoft website.
The software giant said this data will be kept live until "at least 11 July 2007".
It urged users to upgrade to a more secure operating system, such as Windows XP, as soon as possible.
This could mean buying a new PC for those running a machine bought eight years ago with Windows 98 pre-installed. This is because the hardware specifications for Windows 98 fall far short of the minimum required to run Windows XP.
It may also mean they have to replace or update any peripherals they use with that ageing machine.
The turn off for Windows 98 was originally due to fall in late 2003.
However, protests from many developing regions of the world where, at that time, the software was widely used prompted Microsoft to reverse its decision.
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