Why can’t you remove it, what errors are you receiving ?

What Operating System are you using ? is it up to date ?
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.

The fact that it reappears would indicate there are other elements restoring it or downloading it again.

Do you have a firewall and if so what is it ?

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.