Had it happen to me one time too. I went to sleep and left the computer on. The fucker started playing my cd-rom and then blabbing some stuff on his mic (which was heard on my speakers)
I turned it off. next day I ran Trojan Remover and it turned out to be a netbus trojan. That was around 2 years ago.
Now I have an ole' p200 with FreeBSD on it and IPFILTER_DEFAULT_BLOCK compiled into apache. The server is running in extremely high security mode and has all ports closed, and denying all sorts of packets (ICMP, etc). It is then routing the connection to my other 2 comps through a hub. Works like a charm. One thing was that it took me all day long to set it up. But now i'm enjoyin it
12:24AM up 12 days, 1:14, 1 user, load averages: 0.00, 0.00, 0.00