After excluding a dozen processes or so from FSS and including them to Behavior shield:
C:\Archivos de programa\Creative\VoiceCenter\AndreaVC.exe
C:\Archivos de programa\Digital Line Detect\DLG.EXE
C:\Archivos de programa\HP\Digital Imaging\bin\hpqste08.exe
C:\Archivos de programa\Logitech\Logitech WebCam Software\LWS.exe
C:\Archivos de programa\Intel\Intel Matrix Storage Manager\IAANTMON.EXE
C:\Archivos de programa\Intel\Intel Matrix Storage Manager\IAAnotif.exe
C:\Archivos de programa\TrippLite\PowerAlert\console\pastatus.exe
C:\Archivos de programa\TrippLite\PowerAlert\engine\pal.exe
C:\Archivos de programa\UPHClean\uphclean.exe
C:\Archivos de programa\Archivos comunes\Creative Labs Shared\Service\CREATIVELICENSING.EXE
C:\Archivos de programa\Archivos comunes\LogiShrd\LQCVFX\COCIMANAGER.EXE
C:\Archivos de programa\Archivos comunes\LogiShrd\LVMVFM\LVPRCSRV.EXE
C:\Documents and Settings\Hernan Perez\Configuración local\Temp\clclean.0001.dir.0000
C:\Documents and Settings\Hernan Perez\Configuración local\Temp\clclean.0001
C:\Documents and Settings\Hernan Perez\Configuración local\Temp\clclean.0001.dir.0000\~df394b.tmp
C:\Documents and Settings\Hernan Perez\Configuración local\Temp\clclean.0001.dir.0000\~efe2.tmp
C:\WINDOWS\STSYSTRA.EXE
C:\WINDOWS\system32\CTMBHA.dll
C:\WINDOWS\SYSTEM32\CTSVCCDA.EXE
C:\WINDOWS\SYSTEM32\DLA\DLACTRLW.EXE
C:\WINDOWS\SYSTEM32\NVSVC32.EXE
I got stuck with
C:\Documents and Settings\Hernan \Local configuration\Temp\clclean.0001.dir.0000
Did not want to go away. However; This morning fooling around with the Clound Services and streaming updates, I disabled it rebooted and enabled it again and rebooted. Did not get connection stablished or the folder for the steaming updates in Avast! def file that I was expecting to accomplish with it, but I got FSS to work the right way

. Avast! icon is not spinning like crazy any more and last file scanned changes accordingly. Now I have to take back all the exclutions in FSS and see what happens.
@ Tetsou Sorry if you have already clarified this but during the "FSS constant scan" what's the behavior of the tray icon? is it spinning?
I'm asking because I have to check my father's laptop (XP Pro SP3).
Yes, The icon is constantly spinning if the comp is at idle. You can also check your CPU for spikes. Mine was all the time jumpping to 40% o 60% when normally it did not pass 15%. Also take a look at your FSS. See my screenshots. One is FSS working like crazy the other is FSS working like it is supposed to. Guess which is which ?
@ DavidRI had some cpu spikes as explain above