Its due to the RPC/DCOM exploit, which is a vulnerability that allows an attacker to gain access to the destination machine by sending a malformed packet to the DCOM service. It uses the RPC TCP port 135..
You can disables the pop-up in avast but still having the protection
It's Avast Network Shield's notification . It protects the computer from the attacks of Internet worms (e.g. Blaster, Sasser, etc.). It will show warning messages in the system area (above the system clock) every time it detects an Internet worm attack. There is a logging feature too: all attacks will be recorded into a log file so that you can inspect their history, frequency, etc. The llast detected attacks will be displayed on the Last attacks page
