avast Network Shield has blocked a DCOM exploit attack and is simply informing you of it, no further action is required. You can however, switch off the warning/notification message. Open the Network Shield provider and click Customise... button and untick the Show warning messages.
You might want to ensure that Sygate is stealthing your computer (try the one at grc.com), if they can't see your computer it is difficult to attack it.
