You can leave the quarantined item alone (the one in the virus chest of Avast!). It cannot do any harm to leave it there. You can delete it if you like but that is if you're sure that the system is running fine and that it is not a false positive.
Downloaded, registered and ran Avast anti-virus program.
It detected a Trojan and recommended I put it in the virus chest.
What now? Dell Dimension 4600 running XP SP3.