Hi Guntash,
Previously welcome to avast forum,
At the first time installed, whether avast installed properly and smoothly?
RPC is a Remote Procedure Call which control network connections, i think the main problem is not come from avast antivirus but you could check your existing system either to restore or repair your Operating System to fix it first.
You could go to check one forum which discussed same problem with yours : //answers.yahoo.com/question/index?qid=20070912055816AAhtgiu
Hopefully could support you as well.
Regards,
Yanto Chiang