Simply being an administrator doesn't necessarily cut it. Please follow the installation guide carefully and you'll be OK. I know the permissions stuff is about 18 pages of detailed instructions but it really is necessary.
The applicaitons team will be releasing a permissions wizard to help automate the vast majority of this for the installation and service accounts that will make this a lot easier and less error prone. Be watching here and AnswerMonkey.net for that in the coming week or two.
