Dear adkel,
Avast finds and cleans the virus Swen. The arriving messages doesn't mean your computer is infected. They are sent from infected computers of other people, having you e-mail address in their address books. They will keeping arrive until those people clean their computers as well.
The only possibility to rid those infected mails is to filter them out on your mailserver. Some Internet providers have virus filtering capability, but to switch it on is usually up to users (the providers cannot take the responsibility to delete a user's mail without his/hers express permission).