What is 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 icon), Warning section, this contains information on all avast detections.
This is most likely that there are other elements of the malware that are restoring the file after deletion. Deletion isn't really a good first option (you have none left), 'first do no harm' don't delete, send virus to the chest and investigate. Whilst this doesn't seem to be a problem in this case it isn't a god habit to get into.
If you haven't already got this software (freeware), download, install, update and run it, preferably in safe mode.
1.
Ewido, a.k.a. avg anti-spyware If using winXP. or
a-Squared free if using win98/ME.
