What is the event id in Event Viewer for lock, unlock for a computer in Windows XP, Windows 7, Windows Vista and Windows Server 2008?
You will need to enable logging of these events. Do so by opening the group policy editor:
run -> gpedit.msc
and configuring the following category:
Computer Configuration ->
Windows Settings ->
Security Settings ->
Advanced Audit Policy Configuration ->
System Audit Policies - Local Group Policy Object ->
Logon/Logoff ->
Audit Other Login/Logoff Events
(In the Explain tab it says "... allows you to audit ... Locking and unlocking a workstation".)