Windows Vista SP1 includes an evaluation expiration date and a limited license that both expire on a pre-arranged schedule. This schedule is stated in the Microsoft Software License Terms of the Windows Vista SP1 update package. The evaluation period is for three months, and then the limited license expires. When the limited license expires, you receive a warning message to uninstall the service pack or to install a valid Windows Vista license.
“Starting July 1, 2008: Every time that the computer starts, you receive the following message: Your Windows license will expire in 0 hours and Windows will stop working. Back up your files, and then install any edition of Windows Vista. After 2 hours, the Windows Vista prerelease system stops functioning without any additional warning. Additionally, Windows Vista performs one of the following actions, and any unsaved work is lost: • Restart • Display the following message: END_OF_NT_EVALUATION_PERIOD,” Microsoft informed.
But users will still be able to access the pre-release versions of Vista SP1 for a total of two hours before the operating system's kernel will stop working, display the error message and reboot. The true kill-off date for Vista SP1 Beta is July 15, while Release Candidate builds will live to see October 15.
“License expires and computer access is lost: After the license expiration dates that are listed here (This date is October 15 for most users), you can no longer log on to Windows Vista. Additionally, you lose all access to the computer. You must install a valid copy of Windows Vista. To do this, start the installation by starting the computer from the Windows Vista DVD,” Microsoft added.
More info: Microsoft KB954753