Eventviewer eventid for lock and unlock

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".)

The lock event ID is 4800, and the unlock is 4801. You can find them in the Security logs. You probably have to activate their auditing using Local Security Policy (secpol.msc, Local Security Settings in Windows XP) -> Local Policies -> Audit Policy. For Windows 10 see the picture below.

Look in Description of security events in Windows 7 and in Windows Server 2008 R2 under Subcategory: Other Logon/Logoff Events.