Sorry, but it seems you don't understand.
avast! is not accusing your site of anything - your browser tries to connect to the infected site when you open your site, and that connection to the infected site is what's being blocked by avast!.
So, either your site is infected, or some of the linked sites sometimes redirect to the infected site (doesn't have to be every time, could be only sometimes, also based on geoip or any other conditions), or your dns server is routing you somewhere it shouldn't - or maybe your machine itself is compromised. In any case, something is navigating your browser to the bad site.