Securing your network:
http://www.practicallynetworked.com/sup ... secure.htm
(of course, consult your manual for specifics on what's where.)
Securing your laptop:
Well, that will depend on the specific laptop - what model/revision, what OS?
Generally, password-protect the Administrator account (always good practice, anyways) and your own account. Use good, strong passwords. Don't have the Guest account enabled unless you absolutely need it.
Switch the logon screen off (use 'classic' logon page - this requires both username and password to log on.)
Set a BIOS boot password if you're really paranoid. Same goes for a HDD lock (if your drive supports that.)
If you have a fingerprint reader, use it.
Edit: don't leave File & Print Sharing enabled while on a public network (Vista makes this easier, as it allows per-network settings.)