Can you say what is the infected file name, where was it found (C:\windows\system32\infected-file-name.xxx)?
What avast! version and virus database are you using? (see About dialog of avast!)
Maybe you can use full computer on-line scanning of BitDefender for free removal of the malware. Or use AVG Antispyware; SUPERantispyware and/or Spyware Terminator to scan for spywares and trojans. If any infection is detected, better and safer is send the file to Quarantine than to simple delete than.
