You need to go into "tools" then "internet options" then "browsing history" and delete all your cookies. You then need to try again. Unfortunately it may lose all passwords to your favourite sites (like this one) and you'll have to put them in again but I'm afraid it's the only way I know to cure the problem.
Janey - I had cleared my cookies out too and the same thing happened to me. You're not going to believe this but it's happening again this afternoon so I'm going to take the hint and log out completely
If I remember rightly there was another way but it was a pain in the bum. Go to log in and choose option 3, TV customer. Put in an old order number and your name which takes you to a confirmation page. Then yet another page gives you the option to automatically log in. This should keep you logged in if you drop in and out but you will have to repeat this every time you actually log out. Hope that makes sense.