Microsoft is expected to make a "major" announcement after the market closes Monday.
Speculation about the Los Angeles media event has ranged from an unveiling of a new tablet computer that uses low-power chips to a new system that will use an upcoming version of Windows to help people access TV shows and movies across a range of devices.
Microsoft announced a $300 million, 17.6 percent stake in Barnes & Noble's Nook e-book business in April, so a tablet-focused update is also expected.
The Windows 8 system is expected to be released in September or October, in time for devices that will ship for the holiday season. Windows 8 is considered to be the biggest change in decades to the widely used operating system from Microsoft Corp. It has been designed so that it can run desktop, laptop and tablet computers, giving PC makers a better chance of competing with Apple Inc.'s iPad.
"This will be a major Microsoft announcement _ you will not want to miss it," the company said in an email invite to the media on Thursday. A company spokesman declined to comment further.
The event, set to start at 3:30 p.m. Pacific time, is being kept under such tight wraps that the company has declined to provide its location until Monday.