No doubt it will work if you go back to factory conditions first. But that's a lot of work. Your choice, I'd say, to keep using another antivirus or install Avast on a cleanly installed (legal) Windows 7.
I downloaded avast free antivirus (online version). The installation progress normally upto the window which says "installation in progress. Do not turn off your PC". There is a blue bar which fills up normally. But after the bar fills up the window disappears with no future action. There is no error message or confirmation of successful install. No icons have appeared in desktop or in system tray. Action center says Avast is turned off, but clicking "turn on" doesn't do anything. Programs list in Windows control panel does list avast, not with their trademark orange icon, but with a windows default icon. Operating system is windows 7 ultimate. I had avira and malwarebytes previously, but removed both before installing avast. Tried several times even tried repairing avast. Nothing works, please help.