Slideshow: Windows 7 in Pictures: 10 Cool Desktop Features

The move is part of Microsoft's usual effort to push users into upgrading by shutting down, then restarting, PCs equipped with previews. It's also a less-than-subtle reminder that Windows 7 Beta will expire Aug. 1, when the operating system will stop, well, operating.

Users running Windows 7 Beta can continue to use the new operating system without the sudden interruptions -- the shutdowns come without warning -- by downloading and installing Windows 7 Release Candidate (RC), which Microsoft posted for public preview in early May. Windows 7 RC will be available until Aug. 15.

But because Microsoft blocks installation of the RC when the newer edition recognizes that the PC is running Windows 7 Beta, users must do a "clean install," which scrubs the hard drive of operating system, all data and every application. However, Microsoft did publish instructions on how to work around that block; the fix requires editing of the Windows registry.

The other option for Windows 7 Beta users is to revert to the operating system they were running before installing the preview, presumably Windows Vista or XP. That, however, also requires a clean install of the older operating system.

Last month, Microsoft briefly panicked some Windows 7 Beta users when it mistakenly told them that auto-rebooting would start June 1 rather than tomorrow. It quickly made good on their mistake, and sent follow-on messages to users giving them the correct date.

Windows 7 RC, meanwhile, has a June 1, 2010 expiration date, with the every-two-hour auto-shutdowns starting March 1, 2010. Long before that -- on Oct. 22, in fact -- Microsoft will have launched the final, paid version of Windows 7.

Users can download Windows 7 RC free of charge from this Microsoft site, where they can also obtain a product activation key.