How to reset an iPhone? ITunes won't download the latest version on my laptop?

I'm trying to reset an iPhone 4s that my friend sold me… She doesn't know the passcode and I got told how to restore it cos I don't care about anything on the phone because I'll want to put my stuff on there… But iTunes will not download on my laptop. It gets to 73.7/73.7MB and says "Insufficient Permissions",… ANY IDEA WHAT I CAN DO? I really wanna get this up and working fast )

First go to this link:
http://appldnld.apple.com/...DG/iPhone4, 1_5.1.1_9B206_Restore.ipsw

This will download the latest version for the iPhone 4s - save it to your desktop

Now unfortunately you'll need someone else's computer - this guide is to reduce the time you'll need to borrow it for

Plug your phone into the computer and open iTunes

Click your phone in the left hand menu column

Under some information there should be a button that says restore:

Hold down the shift key, and click that button - it should pop up a file browser window

now navigate to where you saved the file and double click it.

iTunes should then begin to restore your phone.

Hope this helps, Good luck!