There are a few things that you can do to make your files more secure.
First, you should understand that a user on one of the computers on your network can't access a folder on another computer unless the folder is shared (via right-click Sharing and Security ...).
Second, the most important thing to do to make your wireless network secure is to enable WEP 2, which is a form of encryption which makes it very hard to break in to the wireless link.
Third, to make your system(s) more secure, you should disable Guest access (via Control Panel -> User Accounts) and make sure that your firewall is turned on (via Control Panel -> Windows Firewall) and that you have the minimum number of exceptions set for the firewall. If you don't have File Sharing set up as an exception then that's another reason a user on one computer can't see the folders on another computer on your network.
Another thing you should consider, although it's a pain, is to make sure that all the accounts on the system have a password, and that the password is strong enough (e.g., 8 characters, mixture of numbers, letters and special characters).