Windows

re-check the subnet for the ADSL. looks like the problem is with ADSL subnet.

This is the subnet the ADSL supplier gave, i didn't think i could change this.

I am assuming that the client pc's on the lan are using the server as the default gateway.

The server needs to have some routing software on it, IE Internet Connection Sharing otherwise, they will not be able to route to the other network.

Shouldn't your subnet for the 10.0.xx network be 255.255.0.0.

When your clients are pinging the ADSL nic, the ADSL nic is responding out to the Internet because that is the default gateway. You need to build a routing table on your server. This is done with a simple command line statement.
c:\route.exe -p add<network_id> mask <subnet mask> <gateway>
This will tell your server to use its LAN nic to respond to all requests from your LAN.

Well - to belabor a probably moot point, your subnet for the ADSL cannot be 255.255.255.248 if you're using an IP range of 217.40.x.x - because by definition the .248 subnet does not have that many hosts in it. But, if you use exactly what your ISPgave you, then it will be correct, I'm sure. Same thing goes for your 10.0.x.x with 255.255.255.0 subnet - if you use a 255.255.255.0 subnet, then your local subnet is actually 10.0.0.x, not 10.0.x.x. OK - 'nough said.

You do need some routing action going on on your server though, so that it can resolve a route for your workstations between the 10.0.0.x subnet and the 217.40.whatever.x subnet. You can use static routing (can be complicated if you're not TCP/IP knowledgeable despite someone else saying it's simple), Internet Connection Sharing (which is basically simple NAT - it "translates" IP addresses between your private, internal network and the public, external Internet), or a true proxy or firewall such as ISA server, other proxy software or a firewall router. Hope this helps!

I assume that the server is the one with the ADSL NIC.
Your Server has to be setup as a router. In NT you can do that by enabling IP forwarding in the TCP/IP properties. In Windows 2000 this is done by enabling the RRAS service in the administrative tools folder in the control panel.
In additon to the above every client needs to have a Default Gateway address which should point to the Server.
If you need any more help email me.