Looking at the answers and comments to this question, it seems that user log-ins can be monitored by checking for Event ID 4624 in the event logs.
Is there a way (through event logs, group policy, or other mechanism) to get Windows to display an alert (e.g. a popup or message in the notification area) when someone logs into the local machine?
Note: there is this other question (but it is about connecting to a shared folder remotely) and this question (which does not specifically ask for any alert mechanism).