I've seen this happen before.  It is the result of a successful hack.  Refer to the following post for details:

http://www.sqlservercentral.com/forums/shwmessage.aspx?forumid=6&messageid=112198

Anyway, the hacker who is using your box to host...

I've seen a few very frightening things recently.  Hackers are port scanning entire IP ranges for boxes withSQL Server installed.  Then they issue brute force password attacks against the sa...