Based on another web posting - ZoneAlarm Toolbar appears to be the problem.
After following instructions on DEP and changing the Maxima (5.23) folder permissions on my Vista system, I uninstalled (via Control Panel) ZoneAlarm and ZoneAlarm Toolbar. Subsequently, I restored ZoneAlarm (92-102-000), but customized the installation so that the ZoneAlarm Toolbar was not reinstalled. XMaxima andf wxMaxima now work.
Hope this helps.