Microsoft Tuesday spelled out exactly how users of Windows Vista betas and release candidates can shift to the final code, and warned that beginning June 1, preview-equipped PCs will automatically reboot every two hours.

Notifications will start going out to users of Vista Ultimate Beta 2, Release Candidate 1 (RC1) and RC2 of the upcoming expiration, a company spokeswoman said. "Customers will have ample time to back up data and migrate their PCs to the final version of Windows Vista," she said.

Upgrade paths were detailed on Microsoft's Vista site.

The only in-place upgrade route -- one that retains the settings, applications, and data as-is -- is from Vista RC1 using a copy of the US$259 Windows Vista Ultimate upgrade. Users running either Beta 2 or RC2 can upgrade to Ultimate, but they must do a "clean install," a process that overwrites the hard drive and destroys all data on it.

These restrictions differ from the advice that Vista users, including ones identifying themselves as MVPs (Most Valuable Professional), advanced users recognized by Microsoft, were dispensing just two weeks ago. Then, the word was that all three preview versions -- Beta 2, RC1, and RC2 -- would accept an in-place upgrade. Their advice was wrong on another count, for they claimed that only a full edition of Ultimate -- which retails for US$399 -- would handle the chore.

All preview users can upgrade to a different edition of Vista, said Microsoft, by doing a clean install using an upgrade SKU of the appropriate version. To ditch the RC2 of Ultimate for the final of Home Premium, for example, users would purchase and clean install the US$159 Home Premium upgrade.

They can also roll back the machine to an earlier Windows, say Windows XP, with a clean install. Such a move, however, requires a full version of the older Windows OS. Users who can't find their installation disc, or weren't provided one (many OEMs didn't ship a separate Windows XP installation disc, but instead stuck the necessary files on a separate partition of the hard drive) will have to buy a new copy. A full version of Windows XP Home lists for US$199.

To Microsoft's credit, it warned Vista testers of possible future problems. Last year, when it posted beta and RC editions for downloading, it said: "To upgrade, you will need to acquire the final edition of Windows Vista and you may have to do a clean installation." The company also told testers that they wouldn't be able to return to their previous OS without a reinstallation. "Before installing Windows Vista on any computer, please remember to back up all your files," it warned.

Procrastinators will find their Vista preview-equipped PC are crippled starting June 1, said Microsoft. "Users that continue to use pre-release versions of Windows Vista will be able to log in for 2-hour sessions to retrieve data," the company said on its Web site. "After 2 hours of use, the PC will automatically reboot without providing the opportunity to save data. The opportunity to log in normally for these 2 hour sessions will only be available for a limited time."

The company spokeswoman downplayed the impact. "We expect most beta users will have moved to a full version of Windows by this time."