Maybe I'm completely misunderstanding what you're after, but it sounds to me like you're trying to add IP addresses for two different subnets to the same physical interface.
If you want the machine to access the 192.168.10.0 and 126.96.36.199 networks from the same machine, you should have two separate NICs. The 192.168.10.x address on the NIC will only talk to other devices with that same IP scheme, unless you have a bridge with proper routing tables to talk to another subnet with a different IP scheme. The same goes for the 203.124.143.x address. To communicate both directions at the same time, you need separate interfaces for each subnet.
If you just want to have a public hide address for the 192.168.10.x address, or a one-to-one static NAT for it, let the firewall take care of the 203.124.143.x address, not the XP box itself.
Again, sorry if I'm misunderstanding, but this is what I'm getting from your questions.