Microsoft is "ramping down" production of the five-year-old Windows NT 4.0 operating system and will no longer sell volume licences for the product, the company said on Tuesday.
The OS will still be available through retail outlets, said a Microsoft spokesman, and volume users will be covered by the Windows 2000 volume licence.
Users with an installed base of NT 4.0 machines will be able to extend their licences by buying the Windows 2000 volume licence and "exercising downgrade rights" to keep using the older system, said the spokesman.
He could not confirm whether this would be more expensive than current licences, nor whether the licence conditions would differ.
Support for the OS will continue, he said, and regular life-cycle guidelines will be issued to keep users up to date with developments.