Lars Ahlgren, senior marketing manager at Microsoft, said: "We'll now support NT 4.0 users for another year until the end of 2004, as an extension of their existing support agreement with Microsoft."
The software firm had previously intended to stop all support for Windows NT 4.0 Server on 31 December.
Ahlgren said the extension would also provide users with free security fixes until the end of 2004.
Windows NT 4 is still used by thousands of businesses across the UK. Microsoft's decision to extend support still leaves users with a difficult decision on whether to migrate to Windows 2000 or Windows 2003, the next release of Windows NT.
However, analyst firm Gartner warned that any migration to Windows 2000 now would be very short term since development and standard support is due to end in March 2005.
Gartner analyst Andy Butler said: "We would be very sceptical advising users to adopt Windows 2000 now as they would be forced to run it right up until the end of support in 2005."
Instead, Butler said users still running Windows NT 4.0 Server should give Windows 2003 serious consideration. "We are pretty confident that users will not be burnt if they deploy Windows 2003 in the third or fourth quarter of this year."
In a mixed environment where users have some Windows 2000 servers, Butler suggested users move their NT 4.0 installations directly onto Windows 2003 and skip Windows 2000.