Thank you MrRon...
I started WMIDiag and found many errors and anomalies in system32\wbem folder, concerning dll system files.
The problem is that I should re-install Windows XP to solve these problems! Is this worth a risk?
I found that Microsoft System Information isn't working either: surely Speccy gathers infos from the same resources, doesn't it?
Well, thanks indeed for your help!