Get into your screensaver panel and un-check the box that says "on resume, display logon screen".
This issue can be due to corrupted power management. To resolve the issue
reinstall Power management.
To reinstall Power Management in Windows Vista, follow these steps:
1. Click Start, right click on computer.
2. Click on device manager.
3. Click the plus (+) sign next to System Devices.
5. Right-click the ACPI Fixed Features button and select Uninstall.
6. Restart the system.
On restarting the system, Windows will automatically detect and
install Power Management on the computer.