Well I have gone into safe mode, uninstalled avast, re-installed it from original download, got new reg number, now if I ask it to check for updates it says cannot connect to server and freezes.
Do you use a firewall? Which one?
Into the firewall settings, the following programs should be allowed to connect:
ashServ.exe (avast! antivirus service)
ashWebSv.exe (avast! Web Scanner)
ashMaiSv.exe (avast! e-Mail Scanner Service)
ashUpdSv.exe (avast! Update Service)
avast.setup (avast! Update executable)