http://support.microsoft.com/kb/315231
I have done this on every XP box I have had in my home, works fine - still allows for logoff/switch users when needed.
Actually - SP2 for xp when installed disables the autologin even if there is only one active ID. This is an easy and so far, permanent fix, so you don't have to worry about another install of .net (regardless of version) screwing things up.