What Operating System are you using ? is it up to date ?
What avast! version and VPS file (virus database) number, e.g. 0630-2 (see about avast!) ?
What is the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
What is your firewall ?
It may be that there are other elements that are restoring/downloading the file.
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.