A separate account is fine, until your smart-ass niece or nephew comes to visit and cracks your machine! To be safe against this level of threat you need to step it up a notch and use strong encryption. My favorite app of this type is TrueCrypt:
http://www.truecrypt.org/
With TrueCrypt you'll create a virtual encrypted drive that can only be accessed by you. Choose a strong password and I promise, even your smart-ass relatives won't be able to get in.
Now that you've got some secure storage you'll need to install applications into it. For that check out Portable Apps:
http://portableapps.org
In particular you'll be interested in Portable Firefox. Install that into your encrypted drive and you've got a private browser no one else can access.
If you're really paranoid, combine the above with Portable Tor and the TorButton Firefox extension. That will give you some (not perfect) protection against tracing your internet activity back to your physical location.
-nycoder