I think he might have been talking about older versions of Avast.
From Avast website.
"Since version 4.1, avast! includes the Virus Cleaner, a tool designed for a complete removal of most common infections from already-infected computers. So now is avast! capable even to reliably heal (not only detect) the most common malware."

Most virus scanners have some type of Virus Vault or Quarantine system. Not all virus's can be removed or cleaned. So if a virus that can not be cleaned replaces a critical system file with it's own it can be quarantined until you can find a solution.

On a related note..
Recently my Avast portable version ran out so I was looking for a new free portable virus scan engine. I just installed ClamWin Portable on my USB drive and ran it on a computer that was thought to have a virus infection. It found 14 infected files. My other scanner detected none after running ClamWin Portable. I'm not to confident in it just yet though. Maybe someone else has some experience with the Clam Engine. It's easy to run from a CD also, which I like.