> Would this stop the access to the other pcs please?
I don't think access to individual PCs is the problem.
To allow this, you have to proactively right-click on say a folder and select the "Share..." command, then enter some information. Even after doing that, there may be some setting up of the firewall running on the machine to allow the "share" to be visible outside the machine.

To find out if you have any shares, then press the "windows"-R key combination (like ctrl-c to copy, but with the key next to ctrl ). This will bring up the "Run..." dialog, to which you type in "cmd" and press enter.
At the command prompt, enter "net share", and you should see something like this
If there's more than that, then questions need asking.

The real problem is that almost all your information appears in clear text on your intranet as it passes from your machine to the router.
Anyone running something like Wireshark would see that, without having to do anything at all to your machine.
The only things which would be presently inaccessible would be say the banking sites you visit which begin with https (as I mentioned earlier).
If he is using wireshark (or similar), then it's pretty sure that he'll know all your other passwords as well.

> Then, yesterday when she was chatting on msn, she nudged her friend..
> oddly she heard the nudge sound in my sons room so she repeated it.
> Again, she heard the same sound in his room. A third nudge and she heard the sound again in his room.
Is it possible in MSN for you to send the "nudge" sound of your choice, or is it always the same sound?
Coincidence would be easy to argue for a few instances of a stock sound. But a dozen instances of your own unique sound would be much harder to explain, especially if you ask why when they're happening.
Or recruit a trusted "nudge-buddy" who won't mind being nudged several dozen times in quick succession. For extra effect, tap out a message in Morse_code using nudges.