Hi Pandanus,
Have you already run antivirus scans on all three systems (w/updated AV programs)? If you don't have AV on each of your systems then you may want to try an online scan such as
Panda or
Kaspersky. They won't remove the virus, if there is one, but they should tell you where there's a problem. You can then remove any infected systems from the network temporarily so that you do not get cut off by your ISP.