Desktop personas

I use different desktop personas to protect different silos of data from attacks (viruses, trojans, privacy infringement ....) that could occur on another silo. For example, I use Gmail in another desktop than where I generally surf the web. Otherwise, there is always a risk that the Gmail full-identity cookie mixes up with the site's Google Analytics cookie, and that this information gets logged somewhere. I store bitcoins in a completely isolated silo. At the moment, I use Virtualbox to manage these data silos. In fact, this is a bit of overkill, because it would be enough to use different user accounts in the same OS image. Unfortunately, it seems to take much more time to switch user accounts -- you also need to provide a password -- than firing up another virtual machine. Does anybody know of a fast and convenient method or program to just switch user accounts (safely) instead of firing up virtual machines?