AT 6 p.m. PST, the company is set to post for download the new "Release Candidate 2" (RC2) beta builds of SP1 and its x64 editions, company officials confirmed.
SP1 is the underlying technology upon which the x64 releases and Windows Server 2003 R2 build.
It will consist primarily of security fixes akin to those Microsoft delivered with Windows XP Service Pack 2, plus a new Security Configuration Wizard.
The final versions of SP1 and the x64 Windows client and server editions are due in the first half of 2005.
Some industry watchers have pegged April as the likely final delivery date, since that is when Microsoft will be holding its annual WinHEC (Windows Hardware Engineering) conference.
Microsoft officials said there are no major feature changes between the Release Candidate 1 builds of SP1 and the x64 Server editions, which Microsoft made available in December 2004, and RC2.
"The difference between RC1 and RC2 really is one of increased robustness as Microsoft prepares for the final RTM," or release to manufacturing, a company spokeswoman said.