Their DSL line was not responding and we weren't sure they knew, since we support their install of our software across the Internet, so we rang them. To cut a long story short, their ISP had disabled their account because an infected PC on their site was spewing forth out to the Internet.
Wasn't the first time it's happened, incidentally. The ISP normally rings them and tells them what they've done. The message probably got lost in the office this time (they're a school).
Would that more ISPs would do this - especially to home users. Would make them sit up and take notice about the problem.
Wade.