hm. well if you have the time i'd recommend a fresh install of your OS. then you should have no problem with permissions. just backup your wallet.dat first of course (hopefully you already do)! i'm sure there is probably an easier way to solve your problem but i would feel uneasy using an OS previously installed and used by someone else, even a friend. if you can afford it, get a new internal HD and start fresh. why not be as safe as possible?
and for security reasons don't use the internet on your admin user account, if possible. create a standard user account for your daily use and only login as admin when you need to make changes.
Windows is insecure regardless of user account. If you're browsing the web as root on a Linux system, you shouldn't be using Linux.