Are you using Windows XP?
Can you schedule a boot-time scanning?
Start avast! > Right click the skin > Schedule a boot-time scanning.
Select for scanning archives.
Boot.

Access denied - like when you can’t delete a file - means, generally, that the file is in use by another process (program) and cannot be repaired/cleaned/moved/handled by avast!

If a virus is replicant (coming and coming again), you should:

  1. Disable System Restore on Windows XP: http://support.microsoft.com/default.aspx?scid=kb;[LN];310405
  2. Clean your temporary files.
  3. Schedule a boot time scanning with avast as I’ve wrote before.
  4. Use a-squared, Free AVG Antispyware, SUPERantispyware or Spyware Terminator (trojan removers).