Some users are probably still using Windows 7 Release Candidate. In the coming weeks and months though, these preliminary OS versions will stop working, according to a reminder issued by Microsoft TechNet.
Starting from 15 February, you will see a warning message once per day, notifying you that Windows 7 RC will expire soon. After 4 hours, you get another notification, followed by more notifications once per hour. The operating system can still be used normally.
From 1 March 2010 onwards, the system will reboot automatically every 2 hours, besides the task bar notification. However, you can still use the operating system.
Starting from 1 June 2010, Windows 7 Release Candidate expires fully. The operating system only displays a black desktop. You will see an activation message telling you that the used system is not genuine. Windows will reboot every 2 hours.
More information: http://support.microsoft.com/kb/971767