I used to work on X.25 networks, but not with a PC, so I can't answer the question directly.
It depends on how you are connecting to the X.25 network. If you are using an X.25 adapter, typically they come with an API so you can access the network at various levels. They may come with a winsock compatible API so you can easily access them from applications. I'm not sure if the X.25 stack fits into the Windows winsock (as a provider layer?) or whether it is a separate stack. Either way the standard TCP/IP components may need some modification to work with the X.25 network.
If you are planning to access the network at a physical level and need layer 2, 3 etc protocols then that's a whole other ball game.