after upgrading to win 7 from xp, cant even ping

hi guys,

had two xp pc's that were working on a network perfectly
then i upgraded them to win 7 (clean install)
they are connected to a d-link ehternet switch, and my broadband router (vodafone white one) has an ethernet
cable into the switch too.
both pc's have internet, both can ping the router and google
but they cant ping each other...
im using static ip addresses, but also tried dhcp, with the same result
i have turned everything off and on again, and there is no firewall, both pc's are running free avast
and i havent had a problem with free avast like this before
is it more than likely something on the broadband router?
any ideas where i can start looking?