Could you point how to solve this Outlook 2007 error: No default email client or current email client cannot fulfill the message request...?

Try the following :

a) Control Panel > Programs > Microsoft Outlook > Change/Repair Installation.
b)  Go into the Adobe Reader XI (my product) menu and look for edit/preferences.
In the long list of preferences, select "email"
add account, gmail is one of the options
set alternate account (not outlook) as the default email account
save and close preferences
c)
    Click Start, and then click Run.
    In the Open box, type regedit, and then press ENTER.
    In Registry Editor, locate the following subkey in the registry:
    HKEY_LOCAL_MACHINE\Software\Clients\Mail\Microsoft Outlook
    Select the subkey, and then press DELETE.
    Click Yes.
    Quit Registry Editor.
    Start Outlook.
    For Outlook 2000 and Outlook 2002, click the Help menu, and then click Detect and Repair .
    For Outlook 2007, click the Help menu, and then click Office Diagnostics .
    For Office 2010, Click Start, click Control Panel, click Programs and Features, select Microsoft Office 2010 and then click the Change button. Next, click Repair, and then click Continue.
    Follow the instructions on the screen to complete the repair.

Hi

Thank you for the reply by now.
Due some urgents jobs I have to complete I couldn't follow your steps, yet.
By the way,  it looks Adobe Reader, that I really have just installed started causing the trouble, isn't it?

By  reading your suggestions I decided simply run CCleaner  just run "Registry Cleanner => Scan for Issues"
Apparently the problem is solved.