[eeyore]

i think its likely that you have clicked stay logged in (little tick box) which will automatically log you in unless you clear your temp internet files.

on a multi user computer it is good practice to clear everything at the end of each session. - to do this in vista you go to tools and click delete browsing history, then delete all. In xp tools is a sub menu of internet options and you have to select delete cookies, delete temp files, and clear history.