Hi,
You probably have the Screen Saver set to show the logon screen - On Resume Display Logon Screen
is checked.
How to Turn Screen Saver On or Off in Windows 7
http://www.sevenforums.com/tutorials/2111-screen-saver-turn-off.html
How to Enable or Disable Windows 7 Screen Saver Password Protection
http://www.sevenforums.com/tutorials/15322-screen-saver-password-protection-enable-disable.html
Interesting :
How to Restore the Windows Energy Screen Saver Back in Windows 7
http://www.sevenforums.com/tutorials/7222-windows-energy-screen-saver-restore.html
Also check Control Panel - Power Options - on left - Require a Password on Wake-Up - set as desired.
These might help :
Windows 7 Sleep Problem and Hibernate Options - see Troubleshooting section
http://www.computerperformance.co.uk/windows7/windows7\_sleep\_problem.htm
How to Troubleshoot a Vista Sleep Mode Problem and Find a Solution - also applies to Windows 7
http://www.vistax64.com/tutorials/63567-power-options-sleep-mode-problems.html
Sometimes resetting the Power Plans back to the defaults helps and then set them as needed.
How to Restore the Default Power Plan Settings in Windows 7
http://www.sevenforums.com/tutorials/950-power-plan-restore-default-settings.html
How to Change the Power Plan Settings in Windows 7
http://www.sevenforums.com/tutorials/778-power-plan-settings-change.html
How to Change Sleep State Settings in Windows 7
http://www.brighthub.com/computing/windows-platform/articles/60294.aspx
Hope this helps.
Rob Brown - MS MVP - Windows Desktop Experience : Bicycle - Mark Twain said it right.