Microsoft has underlined previous warnings that early versions of Windows Vista will be crippled from this Friday, June 1st.

In pop-up messages, Microsoft has told users to: "Back up important data from Windows now. The Windows license will expire in X days. Back up your files, and then install any edition of Windows Vista."

Unless Vista Ultimate Beta 2, Release Candidate 1 (RC1) and RC2 are upgraded to a legal copy by Friday, machines running those editions will automatically reboot every two hours. This crippled state will continue until Aug 28, at which point the operating system will refuse to boot.

Microsoft began urging pre-release users to update last month, when the company announced that only Vista RC1 could be upgraded without requiring a clean install that would delete all data from the PC's hard drive, and that only the $259 Ultimate Upgrade edition could be used for an in-place upgrade of RC1. Vista Beta 2 and RC2, meanwhile, can be upgraded in a clean install using any Vista Upgrade edition.

Alternately, users can revert back to their previous version of Windows, assuming that they have the original installation CD or OEM-provided restore disc. These people must also resort to a clean install.

"As was noted at the outset of the [Vista] Customer Preview Program, you cannot roll back to the previous operating system installation," said Nick White, a Vista program manager, in an FAQ posted to the Vista team blog. "You will either have to purchase and install the final released edition of Windows Vista or re-install a previous version of Windows."

Users can migrate from a Vista preview by downloading an upgrade from the online Windows Marketplace.