better explain about the "unwelcome guest has used one of our computers to access information"?
Was this "guest" seated at one of the computers or do you mean that you think someone has accessed your home network through your wireless router?
If the "guest" had physical access you need to either remove the possibility of physical access to a computer or you need to enforce passwords and limit shares and locking the computer while not physically sitting there using it.
If you mean you suspect accessing your network through your router you need to set up WPA2 security or whatever the strongest security is that your mix of Operating Systems and network cards can handle.
We have a wireless router at home with 4 computers that use it to access the internet. Our computers are a mix of Windows xp, vista and 7. I think that a unwelcome guest has used one of our computers to access information on some of our other computers. Is there any way to check for this.
Any help would be greatly appreciated !