I just saw a very interesting effect on the WinXP machine I have mentioned above. I was asked to have a look on a file from a USB stick. So, I inserted the stick and MCShield prompted that it was checking it. Soon I was prompted that some malware were detected and moved to the program's quarantine folder. But as soon as the malware files appeared there, Avast signaled that it detected malware in the folder and moved them to its own chest!