You should be able to open the sandbox and clear the application that is in it.
I believe you can also right click on the applications file and change the option to always run sandboxed. That is of course if you have the "Integrate into the right-click context menu..." on the Parameters screen, you can add the virtualization option to the context menu.
Check out the Help Center for general info on the Sandbox, see image.
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You could also try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
Vista, win7 - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
You may need to reboot after the repair.