I am bashing nobody but I found that CCleaner wipes out User Identifications and Passwords when using IE8. But not with Firefox.
It seems a CCleaner problem. I have made extensive tests with Win7 and IE8 without CCleaner. In that environment, the issue does not occur.
The IE8 parameters have to be properly introduced:
1. Tools, Internet Options, Advanced, Empty Internet Files folder when browser is closed unmarked
2. Tools, Internet Options, Content, Autocomplete, Settings, User names and passwords on forms marked.
All the best.