Check how are set your avast! services:
Control Panel > Administrative Tools > Services
I suppose you have to have four avast! services:
C:\Program Files\Alwil Software\Avast4\ashServ.exe (avast! antivirus service)
C:\Program Files\Alwil Software\Avast4\ashUpdSv.exe (avast! Update Service)
C:\Program Files\Alwil Software\Avast4\ashWebSv.exe (avast! Web Scanner service)
C:\Program Files\Alwil Software\Avast4\ashMaiSv.exe (avast! e-Mail Scanner service)
The two first of them should be Automatic loaded and the two last Manually loaded.
Generally, RPC error is generally related to other antivirus present (like David posted) or, at least, firewall blocking.
RPC error may also be because of avast! services being stopped as I've said.
Anyway, RPC error is most likely correct by this procedure:
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove
Then choose Repair function in the pop up window (Repair).