After entering the marketplace in 1996, Windows NT 4.0 - arguably Microsoft's most stable server - will be retired slowly over the next two years.
Microsoft said Windows NT Server 4.0 was being phased out due to "increasing adoption of its successor, the Windows 2000 server family of operating system".
Giga research director Laura DiDio predicted further changes to support for the Microsoft product at a user briefing last month.
She added: "Some people will risk staying with an unsupported platform rather than pay to upgrade."
DiDio also pointed out that the older the platform being upgraded, the more the upgrade would cost.
Microsoft stopped selling NT 4.0 through its Volume Licensing programmes in October 2001. By July 1 2002, NT4 will no longer be available through resellers as a pre-installed OS on new PCs. On July 1 2003, Microsoft said, the NT4 operating systems would cease to be offered through the System Builder channel as well.
Changes in services will mean that from the start of 2003 the company will introduce pay-per-incident charges and will drop premier support. By January 1 2005, Microsoft plans to pull the plug on online support.