Accessing OWA2003 from the external Network

Hi, up until 2 weeks ago, users outside the intranet were able to access there mail through the internet via OWA, now suddenly they are unable to. When they try to get access, they recieve the page cannot be displayed message. However they go as far as being asked to authenticate and there after... the error page. I'm using ISA2004
I did try to install ISA2004 service pack 1 and it said the specified program could not be found.

Anyone have any suggestions.