hello,
are you in avast 4 or 5 ? I guess 4... you're looking for the shields panel

just click on the @ tray icon and that will open it

...from there click on detailed view to see all the shields, you'll see the p2p one, reactivate it from there. I suppose you deactivated it with a right click on the same icon in the task bar...
Now if your on avast5 just open the main panel, and you'll see the p2p shield very quickly...see screen shot