No. A cookie stores information you want to keep on their computer, so the next time they visit your site, you can retrieve the information. It will not give you access to their computer.
Your line of questioning leads me to believe that you may be attempting something less than desirable. I hope this is not the case, but you will not find the answers to such things here, I think. Please give up the idea of accessing a user's data -- just think how pissed you would be if you found out that Microsoft was stealing things off your computer...