Thank you Wolf87 for your reply ! But I want to continue my "quest" and discover with your help what can be done and solve some problems that can occur from time to time.
Actually, I was forced to uninstall KAV Personal Pro 4.0.x and come back to Anti Viral Toolkit Pro 3.5.x because it screws up every two XP boot.
I had to wait for about 4-5 minutes for XP to boot completely: hourglass in the system tray, and I reseted often before I discovered by chance that XP would in the end be loaded corectly!
When disabled everything's fine again. I'm gonna wait for an updated version of KAV to install it again and give it another try...
So, here is the "why?" of my "quest".
Please anyone help me find out !
P.S.: But it may be that KAV 4.0.x is not yet fully compatible with XP as claimed by Kaspersky Labs, ... ?