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.
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.