I know what you're saying but the web just doesn't work like that. The only way I can think of is running an application on the server to poll the users activity or perhaps employ an Applet or ActiveX control. This is an extreme overhead though. The web isn't capable of push technology using ASP; it's the client that requests.
Is this some database cleanup function you're wishing to employ? Perhaps the reason for the need to know client activity at that level could provide us with potential solutions to this requirement.