Can you please post the files <avast>\data\log\error.log and warning.log (if they exist on your system)?
And, do I understand it correctly that when you boot, the avast tray icon has the red mark on it, and when you click it, you get the RPC related error?
If so, can you please check if the "avast! Antivirus" service is running (Control Panel -> Administrative Tools -> Services), and if not, if it can be started?
Thanks
Vlk