I have the exact same problem since downloading Service Pack 1 for Windows 7(Home Premium, 64 bit)
Everytime I restart my pc I am uable to open either Firefox or Internet Explorer. The avast icon in the system tray does not rotate at all. I right click on the avast icon and disable shields and then immediately re-enable shields and everything works fine (both Firefox and IE8 open).
I have to go through this procedure everytime I reboot the computer
P.S I don't know if it has any relevance, but I disabled Avast during the downloading and installation of Windows 7 SP1.