Iffen the kid was cruisin' around at college, he was probably saved by the college's server. Iffen he was doin' it at home he was either lucky or the broadband supplier has a virus killer on the server.

If his PC *was* used by a peer-to-peer or distributed computing group an antivirus scan wouldn't detect it.

In any case, get him both firewall and an antivirus. Norton (Symantec) are good, but so is McAfee, Datafellows, and others.

And see to it that he keeps them up-to-date.