Whilst avast's ashWebSv.exe (Web Shield) is slated as the faulting program, more important is the faulting Module, in this case sip.dll which isn't an avast file as far as I'm aware.
A goggle search for sip.dll returns many hit pointing to a VOIP process, do you use VOIP ?
If so what is it and are there any software updates for it ?
What where you doing at the time of the error ?
I would imagine that you wouldn't want the Web Shield to monitor this VOIP traffic, if that is what you were doing. But why that activity would be on the http protocol port 80 is beyond me.