I just posted this on another forum, but I think it'll be just as useful to you guys, so sorry for the copy-pasting:
I'm posting this (and registered, actually!) to post something that worked for me when it tried to force me to update. Download Resource Hacker, open MSN's .exe file with it, go to Version Info and look for an option labeled 1033--Mine was in a folder named "1". Once in there, edit VALUE ProductVersion to something in the future, I was experimenting so mine is VALUE "ProductVersion", "29.0.8064.206". Despite there being other lines that look like they'd matter, this is the only one I edited, and now I can log in with the old MSN without it nagging me to update!
Hope this helps!
EDIT: If you don't have Messenger Plus, you can just set it to use Windows 2000 compatibility mode. It will disable Plus if you do
have it, though.