The default security settings for Windows Vista and Windows Server 2008 allow a user who is not a member of the local Administrators ...

The default security settings for Windows Vista and Windows Server 2008 allow a user who is not a member of the local Administrators group to install only trustworthy printer drivers, such as those provided with Windows or in digitally signed printer driver packages. You can use Group Policy to enable users to install other printer drivers from trusted print servers or to disable this security setting.