|Close web-browser and delete all the cookie files.|
Best way to do that is Ccleaner as it deletes cookies in all three main web-browsers at the same time (IE, Firefox & Opera), so you don't have to hunt around the settings in each browser separately.
Ccleaner is here:
Install it, make sure cookies is ticked in the cleaner options for all the web-browsers it lists. IE will be listed under the 'Windows' tab, any others will be under 'Applications' tab. Now run the cleaner (button on right hand side).
Please bear in mind this is just one of several possible cures for your problem. It doesn't work for everyone if the cause lies elsewhere.