well... I was totally wrong, the problem wasn't Windows 8 fault or related. Don't know how but a file named "Signatures" exists in the fodler "C:\Users\{username}\AppData\Roaming\Microsoft" and this prevented Outlook to work properly altough no error or warning message was shown to user.
thanks...
Under Windows 8 I'm unable to add a new signature to Outlook 2010, the "new signature" window appears but after entering the name of the signature and pressing the {ok} button nothing happens. Never had a similar problem with Windows 7. Any ideas ?