that's what .htaccess files are good for...
Now, since you go on the net to have your friends look at your computer, you should also consider a firewall. People only need to access your http port, but no other ports on your machine.
You can also make apache listen on a different port and use the virtualhost configuration so that the outside world can see only specific pages. Using a different port avoids virus-related traffic (there is no real danger from it, but why should your computer work for others at all, even if it is only sending 404 replies?)
I have a similar setup based on name rather than port, so if I want somebody to view files, they will be available via a virtualhost config at somename.somedyndns.org, all other files are only accessible via a local hostname or local ip