I think we all run into these kinds of issues...

I recently tried to install/test FProt antivirus on my Win 98SE PC. FProt is pretty highly recommended and is supposed to be very resource friendly. The only problem was that it wouldn't even catch the eicar.com test virus in real-time. At least not on my PC! This is just something I couldn't accept. And to make me feel even worse, while my PC doesn't like FProt, I daily read of scads of other users who have no issues at all and are very happy with it.

In the end, I just chalked it up to my luck!