I solved the problem. I replaced all the services of Windows 7 to its original state, then I changed the firewall settings of Windows 7 on the mode "home network" and the same for the firewall Avast! 5.
I do not yet know the cause of the problem but I'm still looking. I think I have been too restrictive with some settings in Windows. For example, the "Server" service was disabled ... This may perhaps explain part of the problem.
Anyway mails are now scanned by Avast! 5. Thank you for your help and you'll be informed as soon as I found the true cause.
Thank you