Microsoft's free previews of Windows 8 will expire next month, giving users about three weeks to upgrade to a paid copy or face hourly restarts.

All three Windows 8 previews -- the Developer Preview of September 2011, the Consumer Preview of February 2012 and the Release Preview -- expire Jan. 15, 2013.

Windows 8 Release Preview, the third and final of the sneak peeks, appeared May 31, five months before the new OS launched in retail on Oct. 26.

The Tom's Hardware blog first reported on the impending expiration of the previews.

Microsoft long ago spelled out what happens to the previews after Jan. 15.

"You have no right to use the software after the expiration date," stated the Release Preview's end-user license agreement (EULA). "Starting from the expiration date, you may not be able to access any unsaved data used with the software. Any applications you receive through the Windows Store will also cease to be available to you in future versions, unless they are made available for re-download and you re-acquire them. You may not receive any other notice."

The previews will also automatically restart every one or two hours -- it's unclear which, since a Microsoft support document notes both -- and messages will pop up on the screen telling customers that they must upgrade to a paid license.

In fact, a message will appear two weeks before the deadline -- in other words, starting on Jan. 1 -- notifying users that the previews will soon expire.