I ran into that problem installing the codec pack, several other utils had the same issue.
Problem is that the account you are using is in the admin group but is not the Admin.
The Administrator account is required when you make changes that affect all users.
In XP if you were in the Admin group you had all the rights, Vista's is different and the Administrator account is the onlly one that can perform some system wide tasks.
You will need to enable the adminstrator account, it is disabled by default.
I strongly recommend that you set a password as well, the default is blank.
Log in as administrator and install the codec pack.
You can go back to using your normal account afterwards the codecs willl be installed for all users.