how to connect wireless to local network

Hello geeks, I have a wireless network running via a router for my laptop, no problems there.
In another part of the house I have 3 PC's, one on W7 pro, the others on XP pro. The PC's are connected via a switch and one (W7) has a wireless adaptor.
I want to run the PC's as a local network with the possibility to connect to the internet once in a while.
Now the question is what is the best way to set up this type of network.
So far I failed to set up the local network, W7 and XP do not communicate.
I created the same user accounts with the same passwords on all PC's and I set IP's for the PC's in the same range as the wireless network.
The good news is that I have an internet connection on the W7 PC but no local network.
Please help