Jon_T: run regedit.exe, could you please verify that following keys+values are correct:
1) HKEY_CLASSES_ROOT\exefile\shellex\ContextMenuHandlers\snxPluginsShellSvr = "{F4B3B0AA-13D1-4a36-BDA2-2055B0F3D5DE}"
2) HKEY_CLASSES_ROOT\CLSID\{F4B3B0AA-13D1-4a36-BDA2-2055B0F3D5DE} = "snxPluginsShell Class"
3) HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ShellIconOverlayIdentifiers\snxPluginsShell = "{F4B3B0AA-13D1-4a36-BDA2-2055B0F3D5DE}"
if 1+2+3 is ok, then run cmd.exe under Administrator, go to avast5 folder and execute: "regsvr32 snxPlugins.dll", this will re-register sandbox context menu, reboot.
Please let me know what you'll find out...