in forum.asp near the top find this code:
if nDays = "" then
nDays = 30
end if
change the nDays = 30 to nDays = 0 to have it default to Show all topics.
You could have also done a search. I found at least 2 other threads (1 was from just a few days ago), that talked about this very thing and had the answer in them also.
http://forum.snitz.com/forum/link.asp?TOPIC_ID=12687