Verena just Google stat counters ~ there are many out there. Here is a link to Sitemeter . I installed one from there for my first website. Installing the statcounter is easy. You will need to set up an account first & they also send a weekly report about visits ~ etc. to your email.
I'm not sure about the comment part ~ check to see if your website host has this as an additional feature. Most of them do whether they are free or one you pay for.
Hope this helps ~ PM me if any questions ~ I'll try & help.
here is the one that MadMaven uses ... I'm sure they come in all flavors .. if you know of a site that uses a guestbook that you like, from a readability point-of-view ... look at the bottom of the page, and there is likely to be a link to the host\creator of the book