The oldest version of Yahoo messenger i found that would connect is the 5.5 (I'm still using it right now) I like it cause it saves your off-lines till you decide to delete them NOT deleting them when the box is closed. You can find the full install of Yahoo messenger 5.5 here:
http://www.zakie.fsnet.co.uk/ChatRoomsStuf...erdownloads.htm
I recomend the April 14th, 2003 link, the last of the 5.5s and the least bugged.