While watching TV last night, I heard a single beep emit from my Dimension 8400 (received 1/5/05).  The computer and the monitor were both on, but nothing was running in Task Manager.  No error message was displayed.  Thirty or so minutes later, I heard it again.  I'm connected to the internet via a cable modem and wired router, but I have McAfee Security Center running - including the McAfee Firewall.

 

I checked under Control Panel and Sounds and found that the "beep" sounded very similar to - Windows XP Information Bar sound.

 

What could this be?