There is a pre-release build (like a beta) in development right now, see
http://forum.avast.com/index.php?topic=63151.0. That is working on a number of sandbox improvements.
However, using IE6 when IE7 & IE8 have been out for a considerable time, is leaving your system open to vulnerabilities closed and security improvements in bot IE7 & IE8. So I can see why you would want to run IE6 sandboxed.
With XP I would suggest you use DropMyRights for internet facing programs, Browsers, email programs, etc. Or use a limited user account.
See "Browsing the Web and Reading E-mail Safely as an Administrator"
http://blogs.msdn.com/michael_howard/archive/2004/11/18/266033.aspxThis obviously applies to those NT based OSes that have administrator settings, winNT, win2k, winXP. Check Bob's, setup instructions and importantly the dropmyrights.msi file needed as MS have now cleared the MS site.
http://mysharedfiles.no-ip.org/dropmyrights