This can be a bit weird sometimes in the way it behaves and the way passwords are stored I find.
Make sure you have all your log in details handy first.
Tools, Internet Options, then under the "general" tab select delete browsing history and make sure all the tick boxes are checked, with the EXCEPTION of the top "preserve favourites" then delete.
Now close IE8 completely and any other windows that are open.
From W7 start or "run" box type
inetcpl.cpl and press enter
this opens the internet options box again without IE8 being open.
Again go into delete browsing history and make sure all tick boxes are cleared EXCEPT cookies and delete again.
NOw again type inetcpl.cpl and this time go to settings (next to delete history button) and click "view files".
There should be none.
That gives you a clean history and no cookies or data saved so you can start again.