Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
However, I'm not to hopeful that this will work as an uninstall boot an reinstall should have resolved it, so it might well be an issue with the OS RPC files. A google search for RPC communication failed returns many hits.
A forums search also returns many hits for RPC communication failed but most of these offer the same advice, other AVs, Repair, Reinstall, etc. and apart from Repair you have tried the other option.
Do you have a network running on your system ?
What is your firewall ?