I have to agree with
garbanzo. If I just want to surf super-isolated so I can't do nuthin' to nuthin', I can boot a Live Linux CD that uses a ram disk and the HD isn't even mounted. Sandboxie gives good protection while allowing you to do stuff, like download freeware... which takes up a lot of my surfing. I think it's much better than running a resident av that tends to gum up the whole system when all you really want is some protection around the browser(e.g. not allow some BHO to get up to something not nice.)
edit: I'm sure there are many who like the virtual box type of set up. I've tried it a few times but to me it's too involved for what it seems to offer. Sandboxie you run and installer and reboot, then set a couple of options. Plus to run normally all I have to do is close the sandbox and stop the Sandboxie Service. No reboot required. A matter of preference.
edit2: the main drawback to Sandboxie is MS killed the 64 bit implementation. I see nothing similar that's as easy to use for 64 bit. A real bummer.