Installing Updates - Errors

I am having problems installing updates to Vista. I'v read the posts, tried
some of the solutions and none of them work for me.

As recently as 3/12 updates were successfully added (KB905866, KB946041),
but four others fail every time (KB946974, KB946983, KB947801, and KB947945).
Code 57A.

In addition, I try to install/uninstall other software (i.e. iTunes), and
also receive errors... "Could not open key:
HKEY_LOCAL_MACHINE\Software\Classes|Interface\{915 DA835-02FE-4953-92FA-624BDF85AB}\ProxyStubCLsid.
Verify that you have sufficient access to that key, or contact your support
personnel."

I have tried to edit the registry for that specific key, but receive an
error stating access is denied.

Seems like all of my problems relate to admin access rights. I have an
admin account, and a guest account. I do not use the guest account.

Windows Vista Home Professional

Thanks.