You wont see any logon or offs in the Security log unless you turn on auditing for those events. They are off by default - at least on XP Pro.

To turn auditing of logons and logoffs...
1) Pull down the menus START, SETTINGS, CONTROL PANEL, ADMINISTRATIVE TOOLS
2) Select LOCAL SECURITY POLICY
3) Under LOCAL POLICIES section click AUDIT POLICY
4) Double-click AUDIT ACCOUNT LOGON EVENTS
5) Check SUCCESS and FAILURE
6) Click OK to save the settings
7) Do this for AUDIT LOGON EVENTS item as well

The one thing to keep in mind if the user doesn't log out you wont have that event.

Good luck