Ok, well the first (and most obvious) thing to check is that all PC's are on the same WORKGROUP. It honestly doesn't matter if they are are called the default WORKGROUP or not as long as your wireless password is secure. (WPA or WPA2 encrypted, a nice strong passphrase) here is a tutorial if your not sure: Wireless Security: How To Protect Your Network
I would recommend working through this tutorial: File Sharing - Between XP and W7 (and vice versa)
although it says XP to Win 7 it does show how to set up a Win 7 machine, the process for Vista is pretty much identical. As you obviously have some knowledge of networking, the few differences there are (and it really is just a couple of menu's in a slightly different place) shouldn't be too difficult for you to figure out.
Note there are two tutorials there, the 2nd one goes into more depth about setting up 7/Vista.
One you have done that, report back, and we'll go from there.