Interestingly, I have considered another possibility.
Most people know there is a registry edit that allows you to set a windows xp/vista/7 PC to automatically logon using a specific user account and password.
http://www.computerperformance.co.uk/windows7/windows7_auto_logon.htmI've just tried using this edit but *intentionally* entering *invalid* user/password details.
The autologon failed and forced the pc back to the logon screen.
I haven't tested this on a workgroup PC, but its worth a try!