LAN-to-LAN PPTP
Vigor 2830N at each end.

I can ping anything that is not a windows computer - printers, voip phones etc.
I can ping windows machines on the same subnet.
I cannot ping windows machines on the other subnet (i.e through the VPN tunnel)

Any advice?

Probably down to software firewalls on the remote computers.
As you're on a different subnet, it's unlikely that those windows machines will 'trust' you.
Check Windows Firewall and/or any third party AV/Firewall software installed.
The fact that you can ping other devices (printers, etc) indicates that there is no problem with the VPN itself.