just to update you
The problem lies with the ad blocker in Norton's Internet Security 2004.
the problem appears to affect all YaBBSE sites because part of the address being returned as messages are retrieved is interpreted by Norton as a pop-up ad. As an example the address showing for this page is
http://www.yabbse.org/community/index.php?board=140;action=post;threadid=26030;title=Post%2Breply;start=0Norton blocks as a default "adid=" as being an advert and thus appears to block "threadid=26030" when it loads the page. Whatever this refers to is therefore suppressed.
Within Norton NIS it is possible to override the default for any page. With their advice I have found it fairly easy to apply.
Should anyone else report the problem the link below explains how to override the default ad blocker settings.
http://service1.symantec.com/Support/nip.nsf/docid/2003090213361336