What is the malware name, the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
Check the avast! Log Viewer (right click the avast 'a' icon), Warning section, this contains information on all avast detections. C:\Program Files\Alwil Software\Avast4\ashLogV.exe
If it keeps coming back, there is likely to be an undetected or hidden element to the infection that restores or downloads the file again. What is your firewall ?
If you haven't already got this software (freeware), download, install, update and run it,
preferably in safe mode and report the findings (it should product a log file).