Just upgraded from avast 7 to 8 and cannot run a manual scan… When I choose the avast 8 icon from the system tray, I get the user interface with the Home, Security, Maintenance, Market, Recommend, and Support tabs – but when I choose the Security tab NOTHING happens (the other tabs immediately display the appropriate window).
(Sorry, version 8.0 .1483)
try repair
controlpanel > ad remove programs > avast > uninstall > repair option…wait two minutes and reboot
any change?
NO CHANGE
Windows said the repair was complete, but Security tab still doesn’t work. >:(
BTW … Using Windows 7 64-bit
Try a clean install. Follow the instructions here http://www.avast.com/uninstall-utility 
Did a clean install - Security tab works … can get to manual scan
problem resolved / solved
Thank you
No problem!
Glad to hear it’s now working. 8)