Does your machine have any other antivirus or antispyware running? Is it possible that one of the other programs is interfering with the installation of Avast?
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."