How to configure windows server 2008 with two networkcards? Clients do not have internet access

If you want the server to use only one specific NIC for routing Internet-bound traffic, then you'll need to set the default gateway on that NIC and not set it on the other one (i.e. leave the field empty in the TCP/IP settings).

But you can't use both at the same time to balance Internet traffic, and you also can't do that on a per-application basis.

You'll want to add a persistent route to the routing table for the default gateway of the 172. interface with a lower metric than the route that was added for the 10. interface. Example:

C:\>route print
===========================================================================
Interface List
0x1 ........................... MS TCP Loopback interface
0x2 ...00 ff ff ff ff ff ...... Intel(R) 82567LM-3 Gigabit Network Connection
===========================================================================
===========================================================================
Active Routes:
Network Destination        Netmask          Gateway       Interface  Metric
          0.0.0.0          0.0.0.0        10.10.0.1       10.10.0.2       20
          0.0.0.0          0.0.0.0       172.16.7.1     172.16.7.51       30
        127.0.0.0        255.0.0.0        127.0.0.1       127.0.0.1       1
Default Gateway:         10.10.0.1
===========================================================================
Persistent Routes:
  None

I would type the command route -p ADD 0.0.0.0 MASK 0.0.0.0 172.16.7.1 METRIC 1

Source

Re Edit 3:

You need to make the metric lower, not equal. When your routing table has 2 routes with the same metric, windows will load balance across these 2 routes. That means it will send 1 packet out the 172 interface and the next out the 10 interface. This is why you have to delete your other route. Go ahead and make the 172 route persistent with a metric of 1 and you should be all set.