What is the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
Windows in its infinite wisdom protects files in use (even malware) or in system folders, so it is likely that avast! can't delete or move files in use. So schedule boot-time scan (see below) if you have XP, win2k or NT, otherwise boot into safe mode and run an avast scan. This should ensure that the file isn't in use and avast should be able to deal with it.
If you have XP or Win2k, you could enable a boot time scan. Right click the avast icon, select Start avast! Antivirus, Menu, 'Schedule boot-time scan...'
