@ Cazozo.......you said "I have no firewall, no other av or security software, but MSE and deactivated."
I used MSE for a few months and don't recall ever seeing a place inside the GUI for "deactivating". So I totally uninstalled that software via Add/Remove in Control Panel. Maybe there is some little piece of MSE still in the registry or at least still "visible" to the Avast installer/updater that is blocking a clean install/update of Avast5. Why don't you try totally uninstalling MSE from your system and reinstall to see if that resolves any of your issues? Worth a try. After uninstalling MSE, uninstall your Avast 5, Run the aswClear.exe for Avast5 (in safe mode), reboot and then do a clean install of Avast5. MSE just might be a factor in your situation.