If both computers are connected to the ISP via the Belkin, it suggests the problem is with one of the computers. Try turning off firewall(s) in that computer.
Thanks for the reply, Fihart.
All firewalls are off: both computers as well as router. Tried AVG update... says "corrupt binary"; downloaded windows updates from MS... says "failed" at the end of each download. All is still as I originally described.
Any other ideas?