Installer failure and security

Hi to all of you
After having installed thze SP3 to XP i am unable to install programs,
hotfixes. I receive error msg: "error #1601 unable to connect to the server"
and: "No connection to the server could established. error: 0x80080005"
It seems to be a problem of permissions, since some other installing
processes stopped with the hint to set the installer back to "basic". But how
to do?
Thanks for your help