cannot ping through vpn

Hi,

I have a netgear box to box vpn which is working perfectly. I have tested
the connection using xp computers at either end and they can ping each other,
and the routers at each end no problem. Vista machines can ping other xp
machines on the local network and also can ping google for instance, but
cannot ping through the vpn to either the remote router or remote computers.
The request simply times out. I have tried turning the firewall off and
removing AV software. None of these things fix it!

Any suggestions would be greatly appreciated.

Thanks