Win7: Interface not registered

Each time I try to open the file in the download windows in Chrome and/or Firefox, a small dialog box appears with the error "Interface not registered" (on my Italian version, "Interfaccia non registrata").
This problem had been haunting for months the Windows 7 computer I left to my parent.

Moreover, I was not able to have access any more to the user account panel in the control panel, nor to modify any file association etc. To sum up, many small, but important, primitive functions of the systems were blocked.

I came back home and finally solved this, mostly thanks to this investigation on techcrunch.
However, in order to be complete, my solution counts two different steps:

> C:\Windows\SysWOW64\regsvr32 C:\Windows\SysWOW64\actxprxy.dll
> C:\Windows\System32\regsvr32 C:\Windows\System32\actxprxy.dll

In practice I registered the right DLL (32 or 64bit) to the right group of application (32 or 64bit).

I have the impression the problem was caused by downgrading Internet Explorer, or most  after I tried to install new ATI drivers for my HP computer.