Internet Session Error

Hi, i have a network of 100ish users, all running windows xp conected to a windows 2003 domain controller, access to the internet via a vigorpro box.  one of the users has started getting this random message.  she has nothing else open on her pc, but occasionally the message comes up.  if she closes the session she can sometimes get back in to the internet.  other occasions she has to try a number of times.  has anyone seen this before?  any info would be gratefully received.  thanks.
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Is the client computer up to date with Windows Updates?  XP Service Pack 3??
julesnAuthor Commented:
Thanks bcapello for the response.  It turns out that the problem was caused by someone else putting a session limit on our firewall, without telling anyone!

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