On topic of antiviruses, I've tried several at friends computer with XP and leaved it with same Microsoft Security Essentials version as Bersaglio, as it had the least impact on system.
As for my XP, now XP POSready, antivirus induced unavoidable slowdown was a thing I couldn't accept. It's been without antivirus since 2009, and haven't had any problems. Of course, I consider myself quite experienced computer user, and using Firewall, Firefox heavily guarded with NoScript (default to block scripts), RequestPolicy Continued and what not. And sometimes using ClamWin or online Antivirus sites for suspicious downloaded files. I know it's probably vulnerable to some threats, but I'm usually not looking for them.
I'm not advocating for not using antivirus programs, I just wanted to point that at most cases common sense is the best defense.