Time is running out for those using beta versions of Windows Vista with Microsoft planning to start crippling the free pre-release operating systems today.

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

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

However, this will come as no surprise to those using Windows Vista betas - Microsoft starting starting pushing pre-release users to upgrade to a full version in April, when it announced that only Vista RC1 could be upgraded without requiring a clean install that would delete all data from the PC's hard drive. 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, but this could cause problems from some. Anyone reverting back to a prior version of Windows must 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 Microsoft’s Nick White, 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 in the US, although Microsoft has drawn criticism for not offering Martetplace in Europe.

See PC Advisor's review of Windows Vista and discuss the new operating system with other readers in our Windows Vista forum.