RPC errors are often associated with having another AV or remnants left behind, which in your case is strange as you have had avast for a year without this problem. But I will ask the question anyway. Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
You are at risk as the essential elements are not running, ashServ.exe, the main scanning engine.
from the Windows Start button, click Run and type services.msc, from the services interface pop-up find the avast! Antivirus service, it should be running but probably not, it should also be set to Automatic, change that and click Start to start the service, see image.
Have you been keeping the program up to date ?
The latest version is 4.8.1229 and that has a self-defence module that should stop other applications deleting avast files or registry entries, etc.
You need to launch ashDisp.exe as I outlined above, from the avast icon right click and select about avast and let me know what program version number you have ?
In S&D disable the resident protection (Tea Timer) if you have it enabled.