I would assume you are using Netscape 4.x, but... The HTML generated by phpBB is of rather high quality (it's actually XHTML, though I can't validate it right now:
http://validator.w3.org/ seems to be offline). Netscape 4.x had lots of issues with tables and its contents, and from a quick glance of the page code your problem might be just that...
My Mozilla (0.9.7+) doesn't show any problems. You might want to try it: go to
http://www.mozilla.org/releases/ and fetch the last one for your OS. If you need assistance, don't hesitate to contact me...
Best regards
Bruckner