In the Settings table look for the variable "cookiepath" and change it to '/'. Then search for the variable "cookiedomain" and empty it.
Then in the datastore table, delete the row labeled "options".
Then you will need to log into the Admin Control Panel twice.
Also you would get a lot better support on vBulletin.com if you mentioned these things there instead of simply saying "I can't log on, I think it is a cookie problem". Details and lots of them are the key to getting good support. The more you tell us, the easier it is to solve your problems.