Unable to register RPC error

I just downloaded the free Avast. When I tried to register it I got this message: AAVM sub system detected RPC error.
What must I do to register this highly rated product?
Thank you, Ben

http://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=46&group=eng

Any other AV currently installed/recently removed?