With the trends towards virtual computing and AJAX/fully-interactive sites rapidly accelerating, one company has married the 2: Ghost, Inc.  Located at http://g.ho.st/, the product has an "online OS" that is supposed to leave nothing (aside from the history of their website url) on the local machine.  Pricing is free (to 2 GB we think), and a very quick test-run on the Alpha version showed its potential (as well as quite a few bugs).  Since its still Alpha, NOT the thing you want to depend on, but enticing to watch for the future.