Hi,
please read here and use the removal-tool offered on the site:
http://securityresponse.symantec.com/avcenter/venc/data/w32.hllw.gaobot.gen.htmlpause AVast shield for scanning with the tool !!
if it doesn't work, please come back with more details:
what Win do you have ?
where exactly was the infected file found ? (full path/folder/filename; see avast's report/log for this)
What do other scanners say about the file, e.g Trend and KAV (see below; pause AVast shield for scanning with those)
Be sure to apply all Windowsupdates, and secure your IE (disable activeX/scripting except for secure sites etc..)
Further Details via the above board search