Only true virus infection can be repaired, e.g. when a virus infects a file it adds a small part to it, provided that file is one that avast's VRDB would monitor (such as system files, .exe or dlls, etc.) and you have run the VRDB, then it may be possible to repair the file to its uninfected state.
However, for the most part so called viruses, trojans (adware/spyware/malware, etc.) can't be repaired because the complete content of the file is malicious. This leaves you with the safest option, move to the chest and the final option of deletion. When a file is in the chest it can't do any harm and you can investigate the infected warning.
