Bug causing blank window

  achilles123 11:28 22 Jul 2004

After I leave my PC unattended for around 50 minutes a blank window appears on the desktop, about 400 x 300 pixels with a button "CLOSE" at the bottom of it. The PC won't respond to ctrl-alt-del, so I can't find the source. If I click on the button it goes away, but having done what? AdAware, spybot, and Norton antivirus, all latest updates, can't find anything. The system is win 98se, 128 meg, with spybot resident, Norton Internet Security, Mozilla and my ISP's dialler in the system tray.
Any suggestions welcome

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