I have a bravenet counter on my site I know they have guestbooks too so you could check them out... I haven't seen any ads yet but then again I have a major pop up blocker so I never see that stuff, give a try... well the people on my site haven't complained either so I guess it should be ok??
www.bravenet.com or
http://www.bravenet.com/webtools/guestbook/index.php