Your best bet is to visit
Hotscripts.com :: The net's largest PHP, CGI, Perl, JavaScript and ASP script collection and resource web portal and choose one of the MANY MANY guestbook / shout box scripts available there...
They are usually pretty good and most are free...
Other than that, you're looking at a bit of custom scripting in whatever system you are comfortable and that your server will support...
Best of Luck
Mike.