Middleware & Client

Hi,
I am a beginnerin OO, Please provide me a solution/feeedback in object oriented way

The scenario is as following:
I am using an ActiveX.exe as a middleware which connects to the SQL 6.5, (all relevant classes make this component) I need to control the users(clients)connectedt to this middleware. so i have put a function say RegisterUser which adds the users in a collection to maintain the current connections. In the client form from Connection command button this is executed and the user is registered.

Now i want to know when this user is off so that i can decrease the colleciton to reflect the updated connections to middleware. This can be very easily done if i further introduce a function which client must call say in the exit menu which simple remove it from the collection (but not works if client machine is power off say?).

IS this approach OK? Or there is some API that I can use to automatically monitor if the client machine is still running and connected.

Please note that client is connected with its winNt name and i get it from GetUserName api.

Any alternative solution to my problem keeping n-tier application in view.

thanks