I don't really think disk cleanup is going to help. The disk cleanup tool build into microsoft is used to free up space on your harddisk by searching your disk for files that you can safely delete. You don't really want to do this in this case. You will be able to delete the cookies folders etc, but the history of the pages you visit is still going to be there when you use drop down window.

There seem to be a bug with IE8 specailly if you are using beta version. You can read more about it here:
http://social.technet.microsoft.com/forums/en-US/itprovistaie/thread/ecaff28f-e00f-4bab-801a-13d364ebad8d/

I hate anything to do with microsoft. I would suggest just get rid of IE and start using firefox or google chrome. They are superior than IE in every aspect.