System Administrator

I have had my Dell Studio 17 for about a month!! Every so ofton a Dell update box appears, I I click and download, when I try and follow the prompts to install, I get a message about permissions and not being able to do this, contact system administrator/ I have two loging, me the main one and one for my girlfriend. How do I set permissions to enable me to install/uninstall things with out these messages??

Thanks,

James