How effective is the p2p shield?
Well, it scans the activity of P2P programs and the accuracy of detection will be the same of other parts of avast.
But you can add final scanning with ashQuick.exe into your P2P program (which is it?).
I think when working with P2P, the most important piece of protection is doing it using a virtualizer. I use Returnil to virtualize the system partition, and I use the security programs as detectors.
After I download a file, I always open it or try to execute it. If the resident modules of avast! do not detect anything, I scan the file with MBAM and SUPER. If they do not report anything, I suppose the files are clean.
If malware is detected (as it happens frequently) I immediately delete the downloaded files and reboot. I do not waste any time trying the clean the system.
Good security policies